linux-kernel - Re: [PATCH v3] mm: slub: avoid wake up kswapd in set_track

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Message-ID: <6be0c134-b5b0-4ff9-9d2a-4b68f22c3762@suse.cz>
Date: Wed, 27 Aug 2025 10:00:42 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Matthew Wilcox <willy@...radead.org>, yangshiguang1011@....com,
 akpm@...ux-foundation.org, cl@...two.org, rientjes@...gle.com,
 roman.gushchin@...ux.dev, glittao@...il.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, yangshiguang <yangshiguang@...omi.com>,
 stable@...r.kernel.org
Subject: Re: [PATCH v3] mm: slub: avoid wake up kswapd in set_track_prepare

On 8/27/25 07:17, Harry Yoo wrote:
> On Mon, Aug 25, 2025 at 05:42:52PM +0200, Vlastimil Babka wrote:
>> On 8/25/25 14:40, Matthew Wilcox wrote:
>> > On Mon, Aug 25, 2025 at 08:17:37PM +0800, yangshiguang1011@....com wrote:
>> >> Avoid deadlock caused by implicitly waking up kswapd by
>> >> passing in allocation flags.
>> > [...]
>> >> +	/* Preemption is disabled in ___slab_alloc() */
>> >> +	gfp_flags &= ~(__GFP_DIRECT_RECLAIM);
>> > 
>> > If you don't mean __GFP_KSWAPD_RECLAIM here, the explanation needs to
>> > be better.
>> 
>> It was suggested by Harry here:
>> https://lore.kernel.org/all/aKKhUoUkRNDkFYYb@harry
>> 
>> I think the comment is enough? Disabling preemption means we can't direct
>> reclaim, but we can wake up kswapd. If the slab caller context is such that
>> we can't, __GFP_KSWAPD_RECLAIM already won't be in the gfp_flags.
> 
> To make it a little bit more verbose, this ^^ explanation can be added to the
> changelog?
> 
>> But I think we should mask our also __GFP_NOFAIL and add __GFP_NOWARN?
> 
> That sounds good.
> 
>> (we should get some common helpers for these kinds of gfp flag manipulations
>> already)
> 
> Any ideas for its name?
> 
> gfp_dont_try_too_hard(),
> gfp_adjust_lightweight(),
> gfp_adjust_mayfail(),
> ...
> 
> I'm not good at naming :/

Looks like there's already gfp_nested_mask() for these purposes. I'm not
sure if it should be allowing GFP_ATOMIC (thus __GFP_HIGH) as it does
though. Seems to contradict the comment about not exhausing reserves. Wonder
if that was raised during review...

The masking out of __GFP_DIRECT_RECLAIM is specific to the slab case so we
don't need a helper for that (unless we find other users). It could be then
e.g. gfp_nested_mask_noblock() ?