linux-kernel - Re: mm: swapin read-ahead and zram

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Message-ID: <dr2ndbnav6ynrxjixfjjsbe4jr66a3niplzpxcbbt3ztjimwzh@l47amo3k5jt5>
Date: Fri, 15 Aug 2025 15:20:50 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...e.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, 
	Suleiman Souhlal <suleiman@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: mm: swapin read-ahead and zram

On (25/08/15 14:51), Sergey Senozhatsky wrote:
[..]
> For instance, notice how entry 1615265 is read, decompressed, then
> presumably evicted from the memory, and read/decompressed again
> soon after, almost immediately.  Also notice how that entry 1615265
> has already went through this cycle 189 times.  It's not entirely
> clear why this happens.
> 
> As far as I can tell, it seems that these extra zram reads are coming from
> the swapin read-ahead:
>  handle_mm_fault
>   do_swap_page
>    swapin_readahead
>     swap_read_folio
>      submit_bio_wait
>       submit_bio_noacct_nocheck
>        __submit_bio
>         zram_submit_bio
>          zram_read_page
>           zram_read_from_zspool

Sorry, I need to correct myself here, it seems that read-ahead win is 1
pretty much all the time in swap_vma_readahead(), so that's probably not
read-ahead after all.