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

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Message-ID: <f34b8dae-82cb-45d1-837a-a02f0db77760@arm.com>
Date: Tue, 17 Jun 2025 15:08:07 +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 2:56 pm, David Hildenbrand wrote:
> On 17.06.25 11:18, Dev Jain wrote:
>>
>> 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.
>
> I mean, if concurrent freeing of a folio would be possible, the 
> function would be horribly broken :)
Yes, trusting the kernel overlords I have dropped my search : )