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
>
