linux-kernel - Re: [RFC PATCH v4 1/5] mm/readahead: Honour new_order in page_cache_ra

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Message-ID: <pskrpcu3lflo3pgeyfvnifcn7z2o6bsieaclntsbyvefs4ab3a@cyfnf36mccvi>
Date: Fri, 9 May 2025 22:50:32 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, David Hildenbrand <david@...hat.com>, 
	Dave Chinner <david@...morbit.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Kalesh Singh <kaleshsingh@...gle.com>, Zi Yan <ziy@...dia.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org, p.raghav@...sung.com
Subject: Re: [RFC PATCH v4 1/5] mm/readahead: Honour new_order in
 page_cache_ra_order()

> >>  
> > 
> > So we always had a fallback to do_page_cache_ra() if the size of the
> > readahead is less than 4 pages (16k). I think this was there because we
> > were adding `2` to the new_order:
> 
> If this is the reason for the magic number 4, then it's a bug in itself IMHO. 4
> pages is only 16K when the page size is 4K; arm64 supports other page sizes. But
> additionally, it's not just ra->size that dictates the final order of the folio;
> it also depends on alignment in the file, EOF, etc.
> 

IIRC, initially we were not able to use order-1 folios[1], so we always
did a fallback for any order < 2 using do_page_cache_ra(). I think that
is where the magic order 2 (4 pages) is coming. Please someone can
correct me if I am wrong.

But we don't have that limitation for file-backed folios anymore, so the
fallback for ra->size < 4 is probably not needed. So the only time we do
a fallback is if we don't support large folios.

> If we remove the fallback condition completely, things will still work out. So
> unless someone can explain the reason for that condition (Matthew?), my vote
> would be to remove it entirely.

I am actually fine with removing the first part of this fallback condition.
But as I said, we still need to do a fallback if we don't support large folios.

--
Pankaj

[1] https://lore.kernel.org/all/ZH0GvxAdw1RO2Shr@casper.infradead.org/