From: Gregory Price <gourry@gourry.net>
To: Hungyu Lin <dennylin0707@gmail.com>
Cc: akpm@linux-foundation.org, willy@infradead.org, jack@suse.cz,
shr@devkernel.io, linux-fsdevel@vger.kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH v2] mm/page-writeback: avoid 32-bit overflow in dirty threshold shares
Date: Wed, 21 Oct 2026 11:28:03 -0400 [thread overview]
Message-ID: <atjZ6yadzSeqjozA@gourry-fedora-PF4VCD3F> (raw)
In-Reply-To: <20261021060654.27855-1-dennylin0707@gmail.com>
On Wed, Oct 21, 2026 at 06:06:54AM +0000, Hungyu Lin wrote:
> The ratios used by __wb_calc_thresh() are scaled to parts per million,
> but the threshold and ratio operands are unsigned long. On 32-bit
> kernels, their multiplication can wrap before the result is assigned
> to u64.
>
> For example, with a threshold of 25600 pages and the default maximum
> ratio of 1000000, wb_max_thresh becomes 4125 pages instead of 25600.
> The shared threshold and minimum share calculations can overflow in
> the same way, producing incorrect per-writeback dirty limits.
>
> Use a u64 threshold for the three ratio multiplications and div_u64()
> to retain the existing division order and rounding.
>
> Fixes: ae82291e9ca4 ("mm: use part per 1000000 for bdi ratios")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
prev parent reply other threads:[~2026-10-21 15:28 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-21 6:06 Hungyu Lin
2026-10-21 15:07 ` Jan Kara
2026-10-21 15:28 ` Gregory Price [this message]
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