linux-kernel - Re: [PATCH v4 4/5] mm,hugetlb: drop obsolete comment about non-present pte and second faults

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Message-ID: <7a6903d0-0ba1-4536-aea0-08ff511f5149@redhat.com>
Date: Fri, 4 Jul 2025 12:47:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Muchun Song <muchun.song@...ux.dev>, Peter Xu <peterx@...hat.com>,
 Gavin Guo <gavinguo@...lia.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/5] mm,hugetlb: drop obsolete comment about
 non-present pte and second faults

On 30.06.25 16:42, Oscar Salvador wrote:
> There is a comment in hugetlb_fault() that does not hold anymore.  This
> one:
> 
>   /*
>    * vmf.orig_pte could be a migration/hwpoison vmf.orig_pte at this
>    * point, so this check prevents the kernel from going below assuming
>    * that we have an active hugepage in pagecache. This goto expects
>    * the 2nd page fault, and is_hugetlb_entry_(migration|hwpoisoned)
>    * check will properly handle it.
>    */
> 
> This was written because back in the day we used to do:
> 
>   hugetlb_fault () {
>    ptep = huge_pte_offset(...)
>    if (ptep) {
>      entry = huge_ptep_get(ptep)
>      if (unlikely(is_hugetlb_entry_migration(entry))
>          ...
>      else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
>          ...
>    }
> 
>    ...
>    ...
> 
>    /*
>     * entry could be a migration/hwpoison entry at this point, so this
>     * check prevents the kernel from going below assuming that we have
>     * a active hugepage in pagecache. This goto expects the 2nd page fault,
>     * and is_hugetlb_entry_(migration|hwpoisoned) check will properly
>     * handle it.
>     */
>    if (!pte_present(entry))
>            goto out_mutex;
>    ...
>   }
> 
> The code was designed to check for hwpoisoned/migration entries upfront,
> and then bail out if further down the pte was not present anymore, relying
> on the second fault to properly handle migration/hwpoison entries that
> time around.
> 
> The way we handle this is different nowadays, so drop the misleading
> comment.
> 
> Link: https://lkml.kernel.org/r/20250627102904.107202-5-osalvador@suse.de
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Gavin Guo <gavinguo@...lia.com>
> Cc: Muchun Song <muchun.song@...ux.dev>
> Cc: Peter Xu <peterx@...hat.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb