Re: [PATCH 8/8] mm: Drop __GFP_DIRECT_RECLAIM flag if PF_MEMALLOC is set

[Michal Hocko <mhocko@suse.com>]

On Fri 08-08-25 15:12:45, Uladzislau Rezki wrote:
> On Thu, Aug 07, 2025 at 01:58:20PM +0200, Michal Hocko wrote:
> > On Thu 07-08-25 09:58:10, Uladzislau Rezki wrote:
> > > The memory allocator already avoids reclaim when PF_MEMALLOC is set.
> > > Clear __GFP_DIRECT_RECLAIM explicitly to suppress might_alloc() warnings
> > > to make more correct behavior.
> > 
> > Rather than chaning the gfp mask would it make more sense to update
> > might_alloc instead?
> > 
> Hm.. I was thinking about it but decided to drop the __GFP_DIRECT_RECLAIM
> instead just to guarantee a no-reclaim behaviour, as it is written now to
> the flag.
> 
> >From the other hand after this patch we would have some unneeded/dead
> checks(if i do not missing anything). For example:
> 
> [1]
>     WARN_ON_ONCE(!can_direct_reclaim);
>     /*
>      * PF_MEMALLOC request from this context is rather bizarre
>      * because we cannot reclaim anything and only can loop waiting
>      * for somebody to do a work for us.
>      */
>     WARN_ON_ONCE(current->flags & PF_MEMALLOC);
> [2]
>     /* no reclaim without waiting on it */
>     if (!(gfp_mask & __GFP_DIRECT_RECLAIM))
>         return false;
> 
>     /* this guy won't enter reclaim */
>     if (current->flags & PF_MEMALLOC)
>         return false;
> 
> [3]
>     /* Caller is not willing to reclaim, we can't balance anything */
>     if (!can_direct_reclaim)
>         goto nopage;
> 
>     /* Avoid recursion of direct reclaim */
>     if (current->flags & PF_MEMALLOC)
>         goto nopage;
> etc.
> 
> But, yes, might_alloc() can be modified also.

I do not have a _strong_ preference but my slight preference would be to
deal with this in might_alloc. Not sure what other think.

-- 
Michal Hocko
SUSE Labs