Re: [PATCH v4 1/6] mm/memcontrol: make lru_zone_size atomic and simplify sanity check

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Aug 31, 2026 at 02:43:31AM +0800, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> commit ca707239e8a7 ("mm: update_lru_size warn and reset bad lru_size")
> introduced a sanity check to catch memcg counter underflow, which was
> more of a workaround for another bug: lru_zone_size is unsigned, so
> underflow wraps it around and returns an enormously large number, then
> the memcg shrinker loops almost forever as the calculated number of
> folios to shrink is huge. That commit also checked if a zero value
> matches the empty LRU list, so we have to hold the LRU lock, and
> handle the positive and negative deltas separately.
> 
> But later commit b4536f0c829c ("mm, memcg: fix the active list aging
> for lowmem requests when memcg is enabled") already removed the LRU
> emptiness check, so handling the deltas separately is no longer
> needed. And if we just turn it into an atomic long, underflow isn't a
> big issue either,

Why atomic long and not just long?

> and can be checked at the reader side, which is
> called much less frequently than the updater.
> 
> So let's turn the counter into an atomic long and check at the reader
> side instead, which has a smaller overhead. The underflow correction
> is removed: a massive leak of the LRU size counter would indicate
> that something else has gone very wrong, and one should fix that
> leaking site instead. Besides, the updater-side sanity check is
> unlikely to catch the leaking site anyway: if a folio was removed
> without updating the counter while other folios remain on the LRU,
> the WARN only triggers much later, from a likely innocent callsite.

Do you have any data to support your claim that updater-side sanity check is not
that useful? Also can you explain the motivation to move the check from the
update side to reader side?