Re: [PATCH v3] mm: slub: avoid wake up kswapd in set_track_prepare

[Vlastimil Babka <vbabka@suse.cz>]

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@163.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() ?