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

[Yang Shi]

On Mon, May 15, 2023 at 10:26 AM Yuanchu Xie <yuanchu@xxxxxxxxxx> 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@xxxxxxxxxx>

Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>

> ---
>  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
>