linux-kernel - Re: [PATCH] mm: pagemap: restrict pagewalk to the requested range

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Date:   Tue, 16 May 2023 10:10:09 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Yuanchu Xie <yuanchu@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        "Zach O'Keefe" <zokeefe@...gle.com>, Peter Xu <peterx@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: pagemap: restrict pagewalk to the requested range

On Mon, May 15, 2023 at 10:26 AM Yuanchu Xie <yuanchu@...gle.com> wrote:
>
> The pagewalk in pagemap_read reads one PTE past the end of the requested
> range, and stops when the buffer runs out of space. While it produces
> the right result, the extra read is unnecessary and less performant.
>
> I timed the following command before and after this patch:
>         dd count=100000 if=/proc/self/pagemap of=/dev/null
> The results are consistently within 0.001s across 5 runs.
>
> Before:
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 0.0763159 s, 671 MB/s
>
> real    0m0.078s
> user    0m0.012s
> sys     0m0.065s
>
> After:
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 0.0487928 s, 1.0 GB/s
>
> real    0m0.050s
> user    0m0.011s
> sys     0m0.039s
>
> Signed-off-by: Yuanchu Xie <yuanchu@...gle.com>

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

> ---
>  fs/proc/task_mmu.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 420510f6a545..6259dd432eeb 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1689,23 +1689,23 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
>         /* watch out for wraparound */
>         start_vaddr = end_vaddr;
>         if (svpfn <= (ULONG_MAX >> PAGE_SHIFT)) {
> +               unsigned long end;
> +
>                 ret = mmap_read_lock_killable(mm);
>                 if (ret)
>                         goto out_free;
>                 start_vaddr = untagged_addr_remote(mm, svpfn << PAGE_SHIFT);
>                 mmap_read_unlock(mm);
> +
> +               end = start_vaddr + ((count / PM_ENTRY_BYTES) << PAGE_SHIFT);
> +               if (end >= start_vaddr && end < mm->task_size)
> +                       end_vaddr = end;
>         }
>
>         /* Ensure the address is inside the task */
>         if (start_vaddr > mm->task_size)
>                 start_vaddr = end_vaddr;
>
> -       /*
> -        * The odds are that this will stop walking way
> -        * before end_vaddr, because the length of the
> -        * user buffer is tracked in "pm", and the walk
> -        * will stop when we hit the end of the buffer.
> -        */
>         ret = 0;
>         while (count && (start_vaddr < end_vaddr)) {
>                 int len;
> --
> 2.40.1.606.ga4b1b128d6-goog
>