linux-kernel - Re: [PATCH] mm: Optimize TLB flushes during page reclaim

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Message-ID: <135b6d6fad6083bfd11a9dc98fad69756b51c59d.camel@surriel.com>
Date: Wed, 22 Jan 2025 23:17:20 -0500
From: Rik van Riel <riel@...riel.com>
To: Vinay Banakar <vny@...gle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Cc: akpm@...ux-foundation.org, willy@...radead.org, mgorman@...e.de, Wei Xu
	 <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH] mm: Optimize TLB flushes during page reclaim

On Mon, 2025-01-20 at 16:47 -0600, Vinay Banakar wrote:
> 
> This patch instead optimizes the process by batching operations,
> issuing one IPI per PMD instead of per page. This reduces interrupts
> by a factor of 512 and enables batching page submissions to BIO. The
> new approach:
> 1. Collect dirty pages that need to be written back
> 2. Issue a single TLB flush for all dirty pages in the batch
> 3. Process the collected pages for writebacks (submit to BIO)
> 
I see how moving the arch_tlbbatch_flush to
between unmapping the pages, and issuing the
IO could reduce TLB flushing operations
significantly.

However, how does that lead to PMD level
operations?

I don't see any code in your patch that gathers
things at the PMD level. The code simply operates
on whatever folio size is on the list that is
being passed to shrink_folio_list()

Is the PMD level thing some MGLRU specific feature?

> 
> I'd appreciate your feedback on this approach, especially on the
> correctness of batched BIO submissions. Looking forward to your
> comments.
> 
Maybe the filesystem people have more comments on
this aspect, but from a VM perspective I suspect
that doing that batch flush in one spot, and then
iterating over the pages should be fine.

My main quibble is with the changelog, and the
comment that refers to "PMD level" operations,
when the code does not appear to be doing any
PMD level coalescing.

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