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SubjectRe: [PATCH 4/7] mm: use the cached page for filemap_fault
On Thu, Oct 18, 2018 at 04:23:15PM -0400, Josef Bacik wrote:
> If we drop the mmap_sem we have to redo the vma lookup which requires
> redoing the fault handler. Chances are we will just come back to the
> same page, so save this page in our vmf->cached_page and reuse it in the
> next loop through the fault handler.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> mm/filemap.c | 30 ++++++++++++++++++++++++++++--
> 1 file changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 65395ee132a0..5212ab637832 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2530,13 +2530,38 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> pgoff_t offset = vmf->pgoff;
> int flags = vmf->flags;
> pgoff_t max_off;
> - struct page *page;
> + struct page *page = NULL;
> + struct page *cached_page = vmf->cached_page;
> vm_fault_t ret = 0;
>
> max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> if (unlikely(offset >= max_off))
> return VM_FAULT_SIGBUS;
>
> + /*
> + * We may have read in the page already and have a page from an earlier
> + * loop. If so we need to see if this page is still valid, and if not
> + * do the whole dance over again.
> + */
> + if (cached_page) {
> + if (flags & FAULT_FLAG_KILLABLE) {
> + error = lock_page_killable(cached_page);
> + if (error) {
> + up_read(&mm->mmap_sem);
> + goto out_retry;
> + }
> + } else
> + lock_page(cached_page);
> + vmf->cached_page = NULL;
> + if (cached_page->mapping == mapping &&
> + cached_page->index == offset) {
> + page = cached_page;
> + goto have_cached_page;
> + }
> + unlock_page(cached_page);
> + put_page(cached_page);
> + }
> +

Can you factor this out so the main code path doesn't get any more
complex than it already is? i.e. something like:

error = vmf_has_cached_page(vmf, &page);
if (error)
goto out_retry;
if (page)
goto have_cached_page;

-dave.

--
Dave Chinner
david@fromorbit.com

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