linux-kernel - Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of THP

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Date: Tue, 13 Feb 2024 19:36:23 +0100
From: David Hildenbrand <david@...hat.com>
To: Charan Teja Kalla <quic_charante@...cinc.com>,
 gregkh@...uxfoundation.org, akpm@...ux-foundation.org, willy@...radead.org,
 vbabka@...e.cz, dhowells@...hat.com, surenb@...gle.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 # see patch description <stable@...r.kernel.org>
Subject: Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of
 THP

>>> Closes:
>>> https://lore.kernel.org/linux-mm/69cb784f-578d-ded1-cd9f-c6db04696336@quicinc.com/
>>> Fixes: 3417013e0d18 ("mm/migrate: Add folio_migrate_mapping()")
>>> Cc: <stable@...r.kernel.org> # see patch description, applicable to <=6.1
>>> Signed-off-by: Charan Teja Kalla <quic_charante@...cinc.com>
>>
>> 3417013e0d18 went into 5.16.
>>
>> cfeed8ffe55b3 went into 6.6.
>>
>> So only 6.1 is affected.
>>
> 
> I tried to dig why the older kernels don't have this issue. The issue
> persists on the older kernels too, AFAICS. See,
> migrate_page_move_mapping().
> 
> So, The Fixes: tag too is unique for 6.1 kernel. And older LTS kernel
> requires different tag it seems..

We really have to identify which commit it actually broke, and whether 
it was repeatedly fixed and broken again.

Because backporting this to 6.1 might be feasible. Backporting to much 
older kernels possibly not.

> 
>>
>> Isn't there a way to bite the bullet and backport that series to 6.1
>> instead?
> 
> My worry is that, because of merge conflicts, not sure If It can end up
> in inducing some other issues.

I can have a look this/next week. I don't recall if there was any 
particular dependency.

> 
> Although we didn't test THP on older kernels, from the code walk, it
> seems issue persists to me on older to 6.1 kernel, unless I am missing
> something here. So back porting of this series to all those LTS kernels,
> may not be a straight forward?
> 
> So, I am really not sure of what is the way forward here...

Again, if we want to fix this properly, we should first identify the 
commit that actually broke it.

If it predates folios, we'd need different fixes for different stable 
kernels most likely.

The big question are:

1) Is it broken in 5.15? Did you actually try to reproduce or is this
    just a guess?

2) How did you come up with 417013e0d18 ("mm/migrate: Add
    folio_migrate_mapping()")

-- 
Cheers,

David / dhildenb