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Message-ID: <CAHbLzkooj5JPbxkgFNWrxkpjaEzYH++DAFRGr87b7jc_WphYQQ@mail.gmail.com>
Date: Thu, 17 Oct 2024 09:48:19 -0700
From: Yang Shi <shy828301@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org, hughd@...gle.com,
david@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: shmem: improve the tmpfs large folio read performance
On Wed, Oct 16, 2024 at 8:25 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2024/10/17 01:33, Yang Shi wrote:
> > On Wed, Oct 16, 2024 at 8:38 AM Matthew Wilcox <willy@...radead.org> wrote:
> >>
> >> On Wed, Oct 16, 2024 at 06:09:30PM +0800, Baolin Wang wrote:
> >>> @@ -3128,8 +3127,9 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> >>> if (folio) {
> >>> folio_unlock(folio);
> >>>
> >>> - page = folio_file_page(folio, index);
> >>> - if (PageHWPoison(page)) {
> >>> + if (folio_test_hwpoison(folio) ||
> >>> + (folio_test_large(folio) &&
> >>> + folio_test_has_hwpoisoned(folio))) {
> >>
> >> Hm, so if we have hwpoison set on one page in a folio, we now can't read
> >> bytes from any page in the folio? That seems like we've made a bad
> >> situation worse.
> >
> > Yeah, I agree. I think we can fallback to page copy if
> > folio_test_has_hwpoisoned is true. The PG_hwpoison flag is per page.
> >
> > The folio_test_has_hwpoisoned is kept set if the folio split is failed
> > in memory failure handler.
>
> Right. I can still keep the page size copy if
> folio_test_has_hwpoisoned() is true. Some sample changes are as follow.
>
> Moreover, I noticed shmem splice_read() and write() also simply return
> an error if the folio_test_has_hwpoisoned() is true, without any
> fallback to page granularity. I wonder if it is worth adding page
> granularity support as well?
I think you should do the same.
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 7e79b6a96da0..f30e24e529b9 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3111,9 +3111,11 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
>
> for (;;) {
> struct folio *folio = NULL;
> + struct page *page = NULL;
> unsigned long nr, ret;
> loff_t end_offset, i_size = i_size_read(inode);
> size_t fsize;
> + bool fallback_page_copy = false;
>
> if (unlikely(iocb->ki_pos >= i_size))
> break;
> @@ -3127,13 +3129,16 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
> if (folio) {
> folio_unlock(folio);
>
> - if (folio_test_hwpoison(folio) ||
> - (folio_test_large(folio) &&
> - folio_test_has_hwpoisoned(folio))) {
> + page = folio_file_page(folio, index);
> + if (PageHWPoison(page)) {
> folio_put(folio);
> error = -EIO;
> break;
> }
> +
> + if (folio_test_large(folio) &&
> + folio_test_has_hwpoisoned(folio))
> + fallback_page_copy = true;
> }
>
> /*
> @@ -3147,7 +3152,7 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
> break;
> }
> end_offset = min_t(loff_t, i_size, iocb->ki_pos +
> to->count);
> - if (folio)
> + if (folio && likely(!fallback_page_copy))
> fsize = folio_size(folio);
> else
> fsize = PAGE_SIZE;
> @@ -3160,8 +3165,13 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
> * virtual addresses, take care about potential
> aliasing
> * before reading the page on the kernel side.
> */
> - if (mapping_writably_mapped(mapping))
> - flush_dcache_folio(folio);
> + if (mapping_writably_mapped(mapping)) {
> + if (unlikely(fallback_page_copy))
> + flush_dcache_page(page);
> + else
> + flush_dcache_folio(folio);
> + }
> +
> /*
> * Mark the page accessed if we read the beginning.
> */
> @@ -3171,7 +3181,10 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
> * Ok, we have the page, and it's up-to-date, so
> * now we can copy it to user space...
> */
> - ret = copy_folio_to_iter(folio, offset, nr, to);
> + if (unlikely(fallback_page_copy))
> + ret = copy_page_to_iter(page, offset,
> nr, to);
> + else
> + ret = copy_folio_to_iter(folio, offset,
> nr, to);
> folio_put(folio);
> } else if (user_backed_iter(to)) {
> /*
The change seems fine to me.
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