Re: [PATCH v2] shmem: clamp negative used-block samples in statfs

[Baolin Wang]

On 10/28/26 1:18 PM, Hungyu Lin wrote:
> percpu_counter_sum() holds the counter lock while scanning the per-CPU
> values, but local updates can still proceed. Concurrent allocation and
> freeing on different CPUs can therefore produce a negative sample even
> though the number of used tmpfs blocks cannot be negative.
>
> shmem_statfs() subtracts this sample from max_blocks, reporting more free
> blocks than the filesystem's total. With a one-page file being written
> and truncated on separate CPUs, GNU df reports negative used space, for
> example 67108864 total bytes, -28672 used and 67137536 available.
>
> Use percpu_counter_sum_positive() to clamp a negative sample to zero.
> This keeps the existing best-effort accounting in statfs and does not
> change allocation or limit enforcement.
>
> Fixes: 7e496299d4d2 ("tmpfs: make tmpfs scalable with percpu_counter for used blocks")
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>