Re: [PATCH v2 3/3] dax: use common 4k zero page for dax mmap reads
On Thu, Jun 15, 2017 at 04:58:56PM +0200, Jan Kara wrote:
> On Wed 14-06-17 11:22:11, Ross Zwisler wrote:
> > @@ -216,17 +217,6 @@ static void dax_unlock_mapping_entry(struct
> > address_space *mapping,
> > dax_wake_mapping_entry_waiter(mapping, index, entry, false);
> > }
> >
> > -static void put_locked_mapping_entry(struct address_space *mapping,
> > -pgoff_t index, void *entry)
> > -{
> > - if (!radix_tree_exceptional_entry(entry)) {
> > - unlock_page(entry);
> > - put_page(entry);
> > - } else {
> > - dax_unlock_mapping_entry(mapping, index);
> > - }
> > -}
> > -
>
> The naming becomes asymetric with this. So I'd prefer keeping
> put_locked_mapping_entry() as a trivial wrapper around
> dax_unlock_mapping_entry() unless we can craft more sensible naming / API
> for entry grabbing (and that would be a separate patch anyway).
Sure, that works for me. I'll fix for v3.
> > -static int dax_load_hole(struct address_space *mapping, void **entry,
> > +static int dax_load_hole(struct address_space *mapping, void *entry,
> > struct vm_fault *vmf)
> > {
> > struct inode *inode = mapping->host;
> > - struct page *page;
> > - int ret;
> > -
> > - /* Hole page already exists? Return it... */
> > - if (!radix_tree_exceptional_entry(*entry)) {
> > - page = *entry;
> > - goto finish_fault;
> > - }
> > + unsigned long vaddr = vmf->address;
> > + int ret = VM_FAULT_NOPAGE;
> > + struct page *zero_page;
> > + void *entry2;
> >
> > - /* This will replace locked radix tree entry with a hole page */
> > - page = find_or_create_page(mapping, vmf->pgoff,
> > - vmf->gfp_mask | __GFP_ZERO);
>
> With this gone, you can also remove the special DAX handling from
> mm/filemap.c: page_cache_tree_insert() and remove from dax.h
> dax_wake_mapping_entry_waiter(), dax_radix_locked_entry() and RADIX_DAX
> definitions. Yay! As a separate patch please.
Oh, yay! :) Sure, I'll have this patch for v3.
Re: [PATCH v2 3/3] dax: use common 4k zero page for dax mmap reads
On Wed 14-06-17 11:22:11, Ross Zwisler wrote: > When servicing mmap() reads from file holes the current DAX code allocates > a page cache page of all zeroes and places the struct page pointer in the > mapping->page_tree radix tree. This has two major drawbacks: > > 1) It consumes memory unnecessarily. For every 4k page that is read via a > DAX mmap() over a hole, we allocate a new page cache page. This means that > if you read 1GiB worth of pages, you end up using 1GiB of zeroed memory. > This is easily visible by looking at the overall memory consumption of the > system or by looking at /proc/[pid]/smaps: > > 7f62e72b3000-7f63272b3000 rw-s 103:00 12 /root/dax/data > Size:1048576 kB > Rss: 1048576 kB > Pss: 1048576 kB > Shared_Clean: 0 kB > Shared_Dirty: 0 kB > Private_Clean: 1048576 kB > Private_Dirty: 0 kB > Referenced: 1048576 kB > Anonymous: 0 kB > LazyFree: 0 kB > AnonHugePages: 0 kB > ShmemPmdMapped:0 kB > Shared_Hugetlb:0 kB > Private_Hugetlb: 0 kB > Swap: 0 kB > SwapPss: 0 kB > KernelPageSize:4 kB > MMUPageSize: 4 kB > Locked:0 kB > > 2) The fact that we had to check for both DAX exceptional entries and for > page cache pages in the radix tree made the DAX code more complex. > > Solve these issues by following the lead of the DAX PMD code and using a > common 4k zero page instead. As with the PMD code we will now insert a DAX > exceptional entry into the radix tree instead of a struct page pointer > which allows us to remove all the special casing in the DAX code. > > Note that we do still pretty aggressively check for regular pages in the > DAX radix tree, especially where we take action based on the bits set in > the page. If we ever find a regular page in our radix tree now that most > likely means that someone besides DAX is inserting pages (which has > happened lots of times in the past), and we want to find that out early and > fail loudly. > > This solution also removes the extra memory consumption. Here is that same > /proc/[pid]/smaps after 1GiB of reading from a hole with the new code: > > 7f2054a74000-7f2094a74000 rw-s 103:00 12 /root/dax/data > Size:1048576 kB > Rss: 0 kB > Pss: 0 kB > Shared_Clean: 0 kB > Shared_Dirty: 0 kB > Private_Clean: 0 kB > Private_Dirty: 0 kB > Referenced:0 kB > Anonymous: 0 kB > LazyFree: 0 kB > AnonHugePages: 0 kB > ShmemPmdMapped:0 kB > Shared_Hugetlb:0 kB > Private_Hugetlb: 0 kB > Swap: 0 kB > SwapPss: 0 kB > KernelPageSize:4 kB > MMUPageSize: 4 kB > Locked:0 kB > > Overall system memory consumption is similarly improved. > > Another major change is that we remove dax_pfn_mkwrite() from our fault > flow, and instead rely on the page fault itself to make the PTE dirty and > writeable. The following description from the patch adding the > vm_insert_mixed_mkwrite() call explains this a little more: > > *** > To be able to use the common 4k zero page in DAX we need to have our PTE > fault path look more like our PMD fault path where a PTE entry can be > marked as dirty and writeable as it is first inserted, rather than > waiting for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call. > > Right now we can rely on having a dax_pfn_mkwrite() call because we can > distinguish between these two cases in do_wp_page(): > > case 1: 4k zero page => writable DAX storage > case 2: read-only DAX storage => writeable DAX storage > > This distinction is made by via vm_normal_page(). vm_normal_page() > returns false for the common 4k zero page, though, just as it does for > DAX ptes. Instead of special casing the DAX + 4k zero page case, we will > simplify our DAX PTE page fault sequence so that it matches our DAX PMD > sequence, and get rid of dax_pfn_mkwrite() completely. > > This means that insert_pfn() needs to follow the lead of insert_pfn_pmd() > and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set > insert_pfn() will do the work that was previously done by wp_page_reuse() > as part of the dax_pfn_mkwrite() call path. > *** This looks generally fine. Just two small comments below. > @@ -216,17 +217,6 @@ static void dax_unlock_mapping_entry(struct > address_space *mapping, > dax_wake_mapping_entry_waiter(mapping, index, entry, false); > } > > -static void put_locked_mapping_entry(struct address_space *mapping, > -
