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

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Message-ID: <8b8ce450-0300-4eed-a80f-2e8e18fb4e11@arm.com>
Date: Tue, 17 Jun 2025 14:48:01 +0530
From: Dev Jain <dev.jain@....com>
To: David Hildenbrand <david@...hat.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 1:17 pm, David Hildenbrand wrote:
> 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.

Well...pretty sure the file read path is taking the inode->i_rwsem or 
i_lock somewhere,

to synchronize with truncation/reclaim, can't figure out where. Reclaim 
takes the i_lock in __remove_mapping and

then freezes the folio reference, so the read path must lock i_lock 
somewhere.


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