linux-kernel - Re: [PATCH v6 6/9] mm: multigenerational lru: aging

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Date:   Tue, 18 Jan 2022 23:31:07 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging

On Thu, Jan 13, 2022 at 01:02:26PM +0100, Michal Hocko wrote:
> On Thu 13-01-22 02:43:38, Yu Zhao wrote:
> [...]
> > > > The bottom line is I can try various optimizations, e.g., preallocate
> > > > a few buffers for a limited number of page walkers and if this number
> > > > has been reached, fallback to the rmap-based function. But I have yet
> > > > to see evidence that calls for additional complexity.
> > > 
> > > I would disagree here. This is not an optimization. You should be
> > > avoiding allocations from the memory reclaim because any allocation just
> > > add a runtime behavior complexity and potential corner cases.
> > 
> > Would __GFP_NOMEMALLOC address your concern? It prevents allocations
> > from accessing the reserves even under PF_MEMALLOC.
> 
> __GFP_NOMEMALLOC would deal with the complete memory depletion concern
> for sure but I am not sure how any of these allocations would succeed
> when called from the direct reclaim. Some access to memory reserves is
> necessary if you insist on allocating from the reclaim process.
> 
> You can have a look at the limited memory reserves access by oom victims
> for an example of how this can be done.

Thanks. I'll change GFP_KERNEL to __GFP_HIGH | __GFP_NOMEMALLOC.
__GFP_HIGH allows some access to memory reserves and __GFP_NOMEMALLOC
prevents the complete depletion. Basically the combination lower the
min watermark by 1/2, and we have been using them for
add_to_swap_cache().