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 14:40:02 +0530
From: Charan Teja Kalla <quic_charante@...cinc.com>
To: Matthew Wilcox <willy@...radead.org>
CC: <gregkh@...uxfoundation.org>, <akpm@...ux-foundation.org>,
        <vbabka@...e.cz>, <dhowells@...hat.com>, <david@...hat.com>,
        <surenb@...gle.com>, <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

Thanks Matthew!!

On 2/13/2024 2:24 PM, Matthew Wilcox wrote:
> I am deeply confused by this commit message.
> 
> Are you saying there is a problem in current HEAD which this fixes, or
> are you saying that this problem has already been fixed, and this patch
> is for older kernels?

Sorry, I meant this patch is __only for older kernels__. We are seeing
this issue on 6.1 LTS kernel.

At least I am not expecting this issue on the HEAD of the linux-next branch.

Seems the below message is not clear from my side to say that:
a) why this issue won't be seen on the latest kernel and
b) the problems associated with the respective patches in back porting
to LTS branch?

"On the recent kernels, this issues is indirectly getting fixed with the
series[1], to be specific[2].

When tried to back port this series, it is observed many merge
conflicts and also seems dependent on many other changes. As backporting
to LTS branches is not a trivial one, the similar change from [2] is
picked as a fix.

[1] https://lore.kernel.org/all/20230821160849.531668-1-david@redhat.com/
[2] https://lore.kernel.org/all/20230821160849.531668-5-david@redhat.com/"

IOW, the below couple of line is ensuring the proper swap entry is
stored in the tail pages which is somehow missed on the older kernels.

static void __split_huge_page_tail(struct folio *folio, int tail,
 		struct lruvec *lruvec, struct list_head *list)
{
     .............
+	if (folio_test_swapcache(folio))
+		new_folio->swap.val = folio->swap.val + tail;
     .............
}

Thanks.