linux-kernel - Re: [PATCH v3 2/4] mm/util: introduce snapshot

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Message-ID: <492025a7-4132-4944-b55b-90c6d6e40bba@redhat.com>
Date: Wed, 16 Jul 2025 20:18:00 +0200
From: David Hildenbrand <david@...hat.com>
To: Luiz Capitulino <luizcap@...hat.com>, willy@...radead.org,
 akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, shivankg@....com,
 sj@...nel.org, harry.yoo@...cle.com
Subject: Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

On 16.07.25 19:36, Luiz Capitulino wrote:
> On 2025-07-16 06:16, David Hildenbrand wrote:
>> [...]
>>
>>> -dump:
>>> -    __dump_folio(foliop, &precise, pfn, idx);
>>> +    __dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx);
>>
>> Nit that can be cleaned up later on top:
>>
>> We should probably call this
>>
>> __dump_page_snapshot() and then just pass ... the page_snapshot.
>>
>>>    }
>>>    void dump_page(const struct page *page, const char *reason)
>>> diff --git a/mm/util.c b/mm/util.c
>>> index 0b270c43d7d1..f270bf42465b 100644
>>> --- a/mm/util.c
>>> +++ b/mm/util.c
>>> @@ -25,6 +25,7 @@
>>>    #include <linux/sizes.h>
>>>    #include <linux/compat.h>
>>>    #include <linux/fsnotify.h>
>>> +#include <linux/page_idle.h>
>>>    #include <linux/uaccess.h>
>>> @@ -1171,3 +1172,81 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
>>>        return 0;
>>>    }
>>>    EXPORT_SYMBOL(compat_vma_mmap_prepare);
>>> +
>>> +static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
>>> +             const struct page *page)
>>> +{
>>> +    /*
>>> +     * Only the first page of a high-order buddy page has PageBuddy() set.
>>> +     * So we have to check manually whether this page is part of a high-
>>> +     * order buddy page.
>>> +     */
>>> +    if (PageBuddy(page))
>>> +        ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
>>> +    else if (page_count(page) == 0 && is_free_buddy_page(page))
>>> +        ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
>>> +
>>> +    if (folio_test_idle(folio))
>>> +        ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
>>> +}
>>> +
>>> +/**
>>> + * snapshot_page() - Create a snapshot of a struct page
>>> + * @ps: Pointer to a struct page_snapshot to store the page snapshot
>>> + * @page: The page to snapshot
>>> + *
>>> + * Create a snapshot of the page and store both its struct page and struct
>>> + * folio representations in @ps.
>>> + *
>>> + * Note that creating a faithful snapshot may fail if the compound
>>
>> Maybe highlight that this is not really expected to happen, ever.
>>
>>> + * state of the page keeps changing (e.g., due to a folio split). In
>>> + * this case, ps->faithful is set to false, and the snapshot assumes
>>
>> There is no ps->faithful.
> 
> Yes, good catch. This was from an earlier version.
> 
> Is it fine if I fix only this with a follow up patch for Andrew in this
> thread or would you prefer that I post v4 with all the other changes as
> well?

I think the series was part of mm-new, but now I only spot it in 
mm-everything, weird. Maybe because of a conflict with the other 
stable_page_flags() change?

So whatever Andrew prefers, really :)

-- 
Cheers,

David / dhildenb