linux-kernel - Re: [QUESTION] xas_reload() in iter_xarray_populate

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Message-ID: <5bbf39b9-ddfc-4d97-8f08-8bd43223982f@redhat.com>
Date: Tue, 17 Jun 2025 09:47:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, ziy@...dia.com, willy@...radead.org,
 dhowells@...hat.com, hughd@...gle.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, ryan.roberts@....com,
 aneesh.kumar@...nel.org
Subject: Re: [QUESTION] xas_reload() in iter_xarray_populate_pages()

On 17.06.25 07:10, Dev Jain wrote:
> 
> On 26/05/25 12:05 pm, Dev Jain wrote:
>> Hello all,
>>
>> After doing an xas_load() and xas_retry(), we take neither a reference nor a lock
>> on the folio, and we do an xas_reload(). Is this just to reduce the time window
>> for a race?
>>
>> If the above is true, then, there is a negligible window between xas_load() and
>> xas_reload(), because only xas_retry() exists between them, so why to even reload()?
>>
>> Thanks,
>> Dev
> 
> I do not completely remember our discussion in THP Cabal; I recall David Howells maybe
> saying that the folios are already locked, so it is safe to do xas_load and then do
> a folio_get()? Even if we remove the redundant xas_reload(), I still don't understand
> why we won't need xas_reload() at least after folio_get()?

I think the points where

(a) this should go all away soon

(b) there is the expectation that the folios cannot get truncated
     concurrently. So we can do an unconditional folio_get(), don't have
     to check folio->mapping etc.

(c) The xas_reload() seems unnecessary and can be dropped.

-- 
Cheers,

David / dhildenb