On Wed, Jun 10, 2020 at 01:13:15PM -0700, Matthew Wilcox wrote:
> page_is_mergeable() would incorrectly claim that two IOs were on different
> pages because they were on different base pages rather than on the
> same THP. This led to a reference counting bug in iomap. Simplify the
> 'same_page' test by just comparing whether we have the same struct page
> instead of doing arithmetic on the physical addresses.
>
> Signed-off-by: Matthew Wilcox (Oracle) <wi...@infradead.org>
> ---
> block/bio.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 5235da6434aa..cd677cde853d 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -747,7 +747,7 @@ static inline bool page_is_mergeable(const struct bio_vec
> *bv,
> if (xen_domain() && !xen_biovec_phys_mergeable(bv, page))
> return false;
>
> - *same_page = ((vec_end_addr & PAGE_MASK) == page_addr);
> + *same_page = bv->bv_page == page;
> if (!*same_page && pfn_to_page(PFN_DOWN(vec_end_addr)) + 1 != page)
> return false;
> return true;
No, this is also wrong. If you put two order-2 pages into the same
bvec, and then add the contents of each subpage one at a time, page 0
will be !same, pages 1-3 will be same, but then pages 4-7 will all be
!same instead of page 4 being !same and pages 5-7 being !same. And the
reference count will be wrong on the second THP.
But now I'm thinking about the whole interaction with the block
layer, and it's all a bit complicated. Changing the definition
of page_is_mergeable() to treat compound pages differently without
also changing bio_next_segment() seems like it'll cause problems with
refcounting if any current users can submit (parts of) a compound page.
And changing how bio_next_segment() works requires auditing all the users,
and I don't want to do that.
We could use a bio flag to indicate whether this is a THP-bearing BIO
and how it should decide whether two pages are actually part of the same
page, but that seems like a really bad bit of added complexity.
We could also pass a flag to __bio_try_merge_page() from the iomap code,
but again added complexity. We could also add a __bio_try_merge_thp()
that would only be called from iomap for now. That would call a new
thp_is_mergable() which would use the THP definition of what a "same
page" is. I think I hate this idea least of all the ones named so far.
My preferred solution is to change the definition of iop->write_count
(and iop->read_count). Instead of being a count of the number of segments
submitted, make it a count of the number of bytes submitted. Like this:
diff -u b/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
--- b/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -174,9 +174,9 @@
}
static void
-iomap_read_finish(struct iomap_page *iop, struct page *page)
+iomap_read_finish(struct iomap_page *iop, struct page *page, unsigned int len)
{
- if (!iop || atomic_dec_and_test(&iop->read_count))
+ if (!iop || atomic_sub_and_test(len, &iop->read_count))
unlock_page(page);
}
@@ -193,7 +193,7 @@
iomap_set_range_uptodate(page, bvec->bv_offset, bvec->bv_len);
}
- iomap_read_finish(iop, page);
+ iomap_read_finish(iop, page, bvec->bv_len);
}
static void
@@ -294,8 +294,8 @@
if (is_contig &&
__bio_try_merge_page(ctx->bio, page, plen, poff, &same_page)) {
- if (!same_page && iop)
- atomic_inc(&iop->read_count);
+ if (iop)
+ atomic_add(plen, &iop->read_count);
goto done;
}
@@ -305,7 +305,7 @@
* that we don't prematurely unlock the page.
*/
if (iop)
- atomic_inc(&iop->read_count);
+ atomic_add(plen, &iop->read_count);
if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) {
gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
@@ -1090,7 +1090,7 @@
static void
iomap_finish_page_writeback(struct inode *inode, struct page *page,
- int error)
+ int error, unsigned int len)
{
struct iomap_page *iop = iomap_page_create(inode, page);
@@ -1101,7 +1101,7 @@
WARN_ON_ONCE(iop && atomic_read(&iop->write_count) <= 0);
- if (!iop || atomic_dec_and_test(&iop->write_count))
+ if (!iop || atomic_sub_and_test(len, &iop->write_count))
end_page_writeback(page);
}
@@ -1135,7 +1135,8 @@
/* walk each page on bio, ending page IO on them */
bio_for_each_thp_segment_all(bv, bio, iter_all)
- iomap_finish_page_writeback(inode, bv->bv_page, error);
+ iomap_finish_page_writeback(inode, bv->bv_page, error,
+ bv->bv_len);
bio_put(bio);
}
/* The ioend has been freed by bio_put() */
@@ -1351,8 +1352,8 @@
merged = __bio_try_merge_page(wpc->ioend->io_bio, page, len, poff,
&same_page);
- if (iop && !same_page)
- atomic_inc(&iop->write_count);
+ if (iop)
+ atomic_add(len, &iop->write_count);
if (!merged) {
if (bio_full(wpc->ioend->io_bio, len)) {
