Re: [PATCH 01/16] huge tmpfs: fix fallocate(vanilla) advance over huge pages

[Yang Shi <shy828301@gmail.com>]

On Fri, Jul 30, 2021 at 12:25 AM Hugh Dickins <hughd@google.com> wrote:
>
> shmem_fallocate() goes to a lot of trouble to leave its newly allocated
> pages !Uptodate, partly to identify and undo them on failure, partly to
> leave the overhead of clearing them until later.  But the huge page case
> did not skip to the end of the extent, walked through the tail pages one
> by one, and appeared to work just fine: but in doing so, cleared and
> Uptodated the huge page, so there was no way to undo it on failure.
>
> Now advance immediately to the end of the huge extent, with a comment on
> why this is more than just an optimization.  But although this speeds up
> huge tmpfs fallocation, it does leave the clearing until first use, and
> some users may have come to appreciate slow fallocate but fast first use:
> if they complain, then we can consider adding a pass to clear at the end.
>
> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> Signed-off-by: Hugh Dickins <hughd@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

A nit below:

> ---
>  mm/shmem.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 70d9ce294bb4..0cd5c9156457 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2736,7 +2736,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
>         inode->i_private = &shmem_falloc;
>         spin_unlock(&inode->i_lock);
>
> -       for (index = start; index < end; index++) {
> +       for (index = start; index < end; ) {
>                 struct page *page;
>
>                 /*
> @@ -2759,13 +2759,26 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
>                         goto undone;
>                 }
>
> +               index++;
> +               /*
> +                * Here is a more important optimization than it appears:
> +                * a second SGP_FALLOC on the same huge page will clear it,
> +                * making it PageUptodate and un-undoable if we fail later.
> +                */
> +               if (PageTransCompound(page)) {
> +                       index = round_up(index, HPAGE_PMD_NR);
> +                       /* Beware 32-bit wraparound */
> +                       if (!index)
> +                               index--;
> +               }
> +
>                 /*
>                  * Inform shmem_writepage() how far we have reached.
>                  * No need for lock or barrier: we have the page lock.
>                  */
> -               shmem_falloc.next++;
>                 if (!PageUptodate(page))
> -                       shmem_falloc.nr_falloced++;
> +                       shmem_falloc.nr_falloced += index - shmem_falloc.next;
> +               shmem_falloc.next = index;

This also fixed the wrong accounting of nr_falloced, so it should be
able to avoid returning -ENOMEM prematurely IIUC. Is it worth
mentioning in the commit log?

>
>                 /*
>                  * If !PageUptodate, leave it that way so that freeable pages
> --
> 2.26.2
>