linux-kernel - Re: [PATCH] mm: move more expensive part of XA setup out of mapping check

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Message-ID: <6a9e41c1-0608-1ba8-788e-8aa402d8a0cb@kernel.dk>
Date:   Tue, 26 Oct 2021 12:11:18 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: move more expensive part of XA setup out of mapping
 check

On 10/26/21 12:08 PM, Matthew Wilcox wrote:
> On Tue, Oct 26, 2021 at 10:01:20AM -0600, Jens Axboe wrote:
>> The fast path here is not needing any writeback, yet we spend time setting
>> up the xarray lookup data upfront. Move the part that actually needs to
>> iterate the address space mapping into a separate helper, saving ~30% of
>> the time here.
> 
> No objection to this patch, but it did remind me that I never saw an answer to
> https://lore.kernel.org/all/CAHk-=wg_-EwefQ_3Osz4iJxTrTk3tfrV53Z7-jaGg=tm9i5TXg@mail.gmail.com/
> (and nor did lore)

Totally missed that, and yes there could be cases where that lookup is
going to be way too slow. I'll take a look.

-- 
Jens Axboe