linux-kernel - Re: [PATCH v4 19/36] mm: create new codetag references during page splitting

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Date: Wed, 28 Feb 2024 19:28:03 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH v4 19/36] mm: create new codetag references during page
 splitting

On 2/28/24 18:50, Suren Baghdasaryan wrote:
> On Wed, Feb 28, 2024 at 12:47 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> 
>>
>> Now this might be rare enough that it's not worth fixing if that would be
>> too complicated, just FYI.
> 
> Yeah. We can fix this by subtracting the "bytes" counter of the "head"
> page for all free_the_page(page + (1 << order), order) calls we do
> inside __free_pages(). But we can't simply use pgalloc_tag_sub()
> because the "calls" counter will get over-decremented (we allocated
> all of these pages with one call). I'll need to introduce a new
> pgalloc_tag_sub_bytes() API and use it here. I feel it's too targeted
> of a solution but OTOH this is a special situation, so maybe it's
> acceptable. WDYT?

Hmm I think there's a problem that once you fail put_page_testzero() and
detect you need to do this, the page might be already gone or reallocated so
you can't get to the tag for decrementing bytes. You'd have to get it
upfront (I guess for "head && order > 0" cases) just in case it happens.
Maybe it's not worth the trouble for such a rare case.

>>
>>
>> > Every time
>> > one of these pages are freed that codetag's "bytes" and "calls"
>> > counters will be decremented. I think accounting will work correctly
>> > irrespective of where these pages are freed, in __free_pages() or by
>> > put_page().
>> >
>>