Re: [PATCH v2 02/20] dax: Create a range version of dax_layout_busy_page()
On Mon, Aug 17, 2020 at 06:53:39PM +0200, Jan Kara wrote:
> On Fri 07-08-20 15:55:08, Vivek Goyal wrote:
> > virtiofs device has a range of memory which is mapped into file inodes
> > using dax. This memory is mapped in qemu on host and maps different
> > sections of real file on host. Size of this memory is limited
> > (determined by administrator) and depending on filesystem size, we will
> > soon reach a situation where all the memory is in use and we need to
> > reclaim some.
> >
> > As part of reclaim process, we will need to make sure that there are
> > no active references to pages (taken by get_user_pages()) on the memory
> > range we are trying to reclaim. I am planning to use
> > dax_layout_busy_page() for this. But in current form this is per inode
> > and scans through all the pages of the inode.
> >
> > We want to reclaim only a portion of memory (say 2MB page). So we want
> > to make sure that only that 2MB range of pages do not have any
> > references (and don't want to unmap all the pages of inode).
> >
> > Hence, create a range version of this function named
> > dax_layout_busy_page_range() which can be used to pass a range which
> > needs to be unmapped.
> >
> > Cc: Dan Williams
> > Cc: linux-nvd...@lists.01.org
> > Signed-off-by: Vivek Goyal
>
> The API looks OK. Some comments WRT the implementation below.
>
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 11b16729b86f..0d51b0fbb489 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -558,27 +558,20 @@ static void *grab_mapping_entry(struct xa_state *xas,
> > return xa_mk_internal(VM_FAULT_FALLBACK);
> > }
> >
> > -/**
> > - * dax_layout_busy_page - find first pinned page in @mapping
> > - * @mapping: address space to scan for a page with ref count > 1
> > - *
> > - * DAX requires ZONE_DEVICE mapped pages. These pages are never
> > - * 'onlined' to the page allocator so they are considered idle when
> > - * page->count == 1. A filesystem uses this interface to determine if
> > - * any page in the mapping is busy, i.e. for DMA, or other
> > - * get_user_pages() usages.
> > - *
> > - * It is expected that the filesystem is holding locks to block the
> > - * establishment of new mappings in this address_space. I.e. it expects
> > - * to be able to run unmap_mapping_range() and subsequently not race
> > - * mapping_mapped() becoming true.
> > +/*
> > + * Partial pages are included. If end is LLONG_MAX, pages in the range from
> > + * start to end of the file are inluded.
> > */
>
> I think the big kerneldoc comment should stay with
> dax_layout_busy_page_range() since dax_layout_busy_page() will be just a
> trivial wrapper around it..
Hi Jan,
Thanks for the review.
Will move kerneldoc comment.
>
> > -struct page *dax_layout_busy_page(struct address_space *mapping)
> > +struct page *dax_layout_busy_page_range(struct address_space *mapping,
> > + loff_t start, loff_t end)
> > {
> > - XA_STATE(xas, >i_pages, 0);
> > void *entry;
> > unsigned int scanned = 0;
> > struct page *page = NULL;
> > + pgoff_t start_idx = start >> PAGE_SHIFT;
> > + pgoff_t end_idx = end >> PAGE_SHIFT;
> > + XA_STATE(xas, >i_pages, start_idx);
> > + loff_t len, lstart = round_down(start, PAGE_SIZE);
> >
> > /*
> > * In the 'limited' case get_user_pages() for dax is disabled.
> > @@ -589,6 +582,22 @@ struct page *dax_layout_busy_page(struct address_space
> > *mapping)
> > if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> > return NULL;
> >
> > + /* If end == LLONG_MAX, all pages from start to till end of file */
> > + if (end == LLONG_MAX) {
> > + end_idx = ULONG_MAX;
> > + len = 0;
> > + } else {
> > + /* length is being calculated from lstart and not start.
> > +* This is due to behavior of unmap_mapping_range(). If
> > +* start is say 4094 and end is on 4096 then we want to
> > +* unamp two pages, idx 0 and 1. But unmap_mapping_range()
> > +* will unmap only page at idx 0. If we calculate len
> > +* from the rounded down start, this problem should not
> > +* happen.
> > +*/
> > + len = end - lstart + 1;
> > + }
>
> Maybe it would be more understandable to use
> unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1);
> below and avoid all this rounding and special-casing.
Will do.
>
> > +
> > /*
> > * If we race get_user_pages_fast() here either we'll see the
> > * elevated page count in the iteration and wait, or
> > @@ -601,10 +610,10 @@ struct page *dax_layout_busy_page(struct
> > address_space *mapping)
> > * guaranteed to either see new references or prevent new
> > * references from being established.
> > */
> > - unmap_mapping_range(mapping, 0, 0, 0);
> > + unmap_mapping_range(mapping, start, len, 0);
> >
> > xas_lock_irq();
> > - xas_for_each(, entry,
Re: [PATCH v2 02/20] dax: Create a range version of dax_layout_busy_page()
On Fri 07-08-20 15:55:08, Vivek Goyal wrote:
> virtiofs device has a range of memory which is mapped into file inodes
> using dax. This memory is mapped in qemu on host and maps different
> sections of real file on host. Size of this memory is limited
> (determined by administrator) and depending on filesystem size, we will
> soon reach a situation where all the memory is in use and we need to
> reclaim some.
>
> As part of reclaim process, we will need to make sure that there are
> no active references to pages (taken by get_user_pages()) on the memory
> range we are trying to reclaim. I am planning to use
> dax_layout_busy_page() for this. But in current form this is per inode
> and scans through all the pages of the inode.
>
> We want to reclaim only a portion of memory (say 2MB page). So we want
> to make sure that only that 2MB range of pages do not have any
> references (and don't want to unmap all the pages of inode).
>
> Hence, create a range version of this function named
> dax_layout_busy_page_range() which can be used to pass a range which
> needs to be unmapped.
>
> Cc: Dan Williams
> Cc: linux-nvd...@lists.01.org
> Signed-off-by: Vivek Goyal
The API looks OK. Some comments WRT the implementation below.
> diff --git a/fs/dax.c b/fs/dax.c
> index 11b16729b86f..0d51b0fbb489 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -558,27 +558,20 @@ static void *grab_mapping_entry(struct xa_state *xas,
> return xa_mk_internal(VM_FAULT_FALLBACK);
> }
>
> -/**
> - * dax_layout_busy_page - find first pinned page in @mapping
> - * @mapping: address space to scan for a page with ref count > 1
> - *
> - * DAX requires ZONE_DEVICE mapped pages. These pages are never
> - * 'onlined' to the page allocator so they are considered idle when
> - * page->count == 1. A filesystem uses this interface to determine if
> - * any page in the mapping is busy, i.e. for DMA, or other
> - * get_user_pages() usages.
> - *
> - * It is expected that the filesystem is holding locks to block the
> - * establishment of new mappings in this address_space. I.e. it expects
> - * to be able to run unmap_mapping_range() and subsequently not race
> - * mapping_mapped() becoming true.
> +/*
> + * Partial pages are included. If end is LLONG_MAX, pages in the range from
> + * start to end of the file are inluded.
> */
I think the big kerneldoc comment should stay with
dax_layout_busy_page_range() since dax_layout_busy_page() will be just a
trivial wrapper around it..
> -struct page *dax_layout_busy_page(struct address_space *mapping)
> +struct page *dax_layout_busy_page_range(struct address_space *mapping,
> + loff_t start, loff_t end)
> {
> - XA_STATE(xas, >i_pages, 0);
> void *entry;
> unsigned int scanned = 0;
> struct page *page = NULL;
> + pgoff_t start_idx = start >> PAGE_SHIFT;
> + pgoff_t end_idx = end >> PAGE_SHIFT;
> + XA_STATE(xas, >i_pages, start_idx);
> + loff_t len, lstart = round_down(start, PAGE_SIZE);
>
> /*
>* In the 'limited' case get_user_pages() for dax is disabled.
> @@ -589,6 +582,22 @@ struct page *dax_layout_busy_page(struct address_space
> *mapping)
> if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> return NULL;
>
> + /* If end == LLONG_MAX, all pages from start to till end of file */
> + if (end == LLONG_MAX) {
> + end_idx = ULONG_MAX;
> + len = 0;
> + } else {
> + /* length is being calculated from lstart and not start.
> + * This is due to behavior of unmap_mapping_range(). If
> + * start is say 4094 and end is on 4096 then we want to
> + * unamp two pages, idx 0 and 1. But unmap_mapping_range()
> + * will unmap only page at idx 0. If we calculate len
> + * from the rounded down start, this problem should not
> + * happen.
> + */
> + len = end - lstart + 1;
> + }
Maybe it would be more understandable to use
unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1);
below and avoid all this rounding and special-casing.
> +
> /*
>* If we race get_user_pages_fast() here either we'll see the
>* elevated page count in the iteration and wait, or
> @@ -601,10 +610,10 @@ struct page *dax_layout_busy_page(struct address_space
> *mapping)
>* guaranteed to either see new references or prevent new
>* references from being established.
>*/
> - unmap_mapping_range(mapping, 0, 0, 0);
> + unmap_mapping_range(mapping, start, len, 0);
>
> xas_lock_irq();
> - xas_for_each(, entry, ULONG_MAX) {
> + xas_for_each(, entry, end_idx) {
> if (WARN_ON_ONCE(!xa_is_value(entry)))
> continue;
> if (unlikely(dax_is_locked(entry)))
> @@ -625,6 +634,27 @@ struct page *dax_layout_busy_page(struct address_space
>
[PATCH v2 02/20] dax: Create a range version of dax_layout_busy_page()
virtiofs device has a range of memory which is mapped into file inodes
using dax. This memory is mapped in qemu on host and maps different
sections of real file on host. Size of this memory is limited
(determined by administrator) and depending on filesystem size, we will
soon reach a situation where all the memory is in use and we need to
reclaim some.
As part of reclaim process, we will need to make sure that there are
no active references to pages (taken by get_user_pages()) on the memory
range we are trying to reclaim. I am planning to use
dax_layout_busy_page() for this. But in current form this is per inode
and scans through all the pages of the inode.
We want to reclaim only a portion of memory (say 2MB page). So we want
to make sure that only that 2MB range of pages do not have any
references (and don't want to unmap all the pages of inode).
Hence, create a range version of this function named
dax_layout_busy_page_range() which can be used to pass a range which
needs to be unmapped.
Cc: Dan Williams
Cc: linux-nvd...@lists.01.org
Signed-off-by: Vivek Goyal
---
fs/dax.c| 66 -
include/linux/dax.h | 6 +
2 files changed, 54 insertions(+), 18 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 11b16729b86f..0d51b0fbb489 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -558,27 +558,20 @@ static void *grab_mapping_entry(struct xa_state *xas,
return xa_mk_internal(VM_FAULT_FALLBACK);
}
-/**
- * dax_layout_busy_page - find first pinned page in @mapping
- * @mapping: address space to scan for a page with ref count > 1
- *
- * DAX requires ZONE_DEVICE mapped pages. These pages are never
- * 'onlined' to the page allocator so they are considered idle when
- * page->count == 1. A filesystem uses this interface to determine if
- * any page in the mapping is busy, i.e. for DMA, or other
- * get_user_pages() usages.
- *
- * It is expected that the filesystem is holding locks to block the
- * establishment of new mappings in this address_space. I.e. it expects
- * to be able to run unmap_mapping_range() and subsequently not race
- * mapping_mapped() becoming true.
+/*
+ * Partial pages are included. If end is LLONG_MAX, pages in the range from
+ * start to end of the file are inluded.
*/
-struct page *dax_layout_busy_page(struct address_space *mapping)
+struct page *dax_layout_busy_page_range(struct address_space *mapping,
+ loff_t start, loff_t end)
{
- XA_STATE(xas, >i_pages, 0);
void *entry;
unsigned int scanned = 0;
struct page *page = NULL;
+ pgoff_t start_idx = start >> PAGE_SHIFT;
+ pgoff_t end_idx = end >> PAGE_SHIFT;
+ XA_STATE(xas, >i_pages, start_idx);
+ loff_t len, lstart = round_down(start, PAGE_SIZE);
/*
* In the 'limited' case get_user_pages() for dax is disabled.
@@ -589,6 +582,22 @@ struct page *dax_layout_busy_page(struct address_space
*mapping)
if (!dax_mapping(mapping) || !mapping_mapped(mapping))
return NULL;
+ /* If end == LLONG_MAX, all pages from start to till end of file */
+ if (end == LLONG_MAX) {
+ end_idx = ULONG_MAX;
+ len = 0;
+ } else {
+ /* length is being calculated from lstart and not start.
+* This is due to behavior of unmap_mapping_range(). If
+* start is say 4094 and end is on 4096 then we want to
+* unamp two pages, idx 0 and 1. But unmap_mapping_range()
+* will unmap only page at idx 0. If we calculate len
+* from the rounded down start, this problem should not
+* happen.
+*/
+ len = end - lstart + 1;
+ }
+
/*
* If we race get_user_pages_fast() here either we'll see the
* elevated page count in the iteration and wait, or
@@ -601,10 +610,10 @@ struct page *dax_layout_busy_page(struct address_space
*mapping)
* guaranteed to either see new references or prevent new
* references from being established.
*/
- unmap_mapping_range(mapping, 0, 0, 0);
+ unmap_mapping_range(mapping, start, len, 0);
xas_lock_irq();
- xas_for_each(, entry, ULONG_MAX) {
+ xas_for_each(, entry, end_idx) {
if (WARN_ON_ONCE(!xa_is_value(entry)))
continue;
if (unlikely(dax_is_locked(entry)))
@@ -625,6 +634,27 @@ struct page *dax_layout_busy_page(struct address_space
*mapping)
xas_unlock_irq();
return page;
}
+EXPORT_SYMBOL_GPL(dax_layout_busy_page_range);
+
+/**
+ * dax_layout_busy_page - find first pinned page in @mapping
+ * @mapping: address space to scan for a page with ref count > 1
+ *
+ * DAX requires ZONE_DEVICE mapped pages. These pages are never
+ * 'onlined' to the page allocator so they are considered idle when
+ * page->count == 1. A
