linux-kernel - Re: [PATCH 08/13] fs: add read support for RWF

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Message-ID: <Zy5Zb8Twe7QBkHMh@casper.infradead.org>
Date: Fri, 8 Nov 2024 18:33:19 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, hannes@...xchg.org,
	clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/13] fs: add read support for RWF_UNCACHED

On Fri, Nov 08, 2024 at 10:43:31AM -0700, Jens Axboe wrote:
> +++ b/mm/swap.c
> @@ -472,6 +472,8 @@ static void folio_inc_refs(struct folio *folio)
>   */
>  void folio_mark_accessed(struct folio *folio)
>  {
> +	if (folio_test_uncached(folio))
> +		return;
>  	if (lru_gen_enabled()) {

This feels like it might be a problem.  If, eg, process A is doing
uncached IO and process B comes along and, say, mmap()s it, I think
we'll need to clear the uncached flag in order to have things work
correctly.  It's a performance problem, not a correctness problem.