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

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Message-ID: <Ye3LVBuWBcPGd1Es@google.com>
Date:   Sun, 23 Jan 2022 14:40:36 -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 12:57:35PM +0100, Michal Hocko wrote:
> On Wed 12-01-22 16:43:15, Yu Zhao wrote:
> > On Wed, Jan 12, 2022 at 11:17:53AM +0100, Michal Hocko wrote:
> [...]
> > > Is there any reason you are not using folio_memcg_lock in the
> > > pte walk instead?
> > 
> > We have a particular lruvec (the first arg), hence a particular memcg
> > to lock. But we don't have a particular page to lock.
> 
> That is certainly true at this layer but the locking should be needed
> only for specific pages, no?

Yes.

> So you can move the lock down to the
> callback which examines respective pages. Or is there anything
> preventing that?

No.

> To be honest, and that is the reason I am asking, I really do not like
> to open code the migration synchronization outside of the memcg proper.

Agreed.

> Code paths which need a stable memcg are supposed to be using
> folio_memcg_lock for the specific examination time.

No argument here, just a clarification: when possible I prefer to
lock a batch of pages rather than individual ones.

> If you prefer a
> trylock approach for this usecase then we can add one.

Done. Thanks.