linux-kernel - Re: [PATCH] mm: hugetlb: independent PMD page table shared count

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Message-ID: <c9e45248-089e-4041-8f7e-98203a0831cf@redhat.com>
Date: Tue, 17 Dec 2024 12:55:32 +0100
From: David Hildenbrand <david@...hat.com>
To: jane.chu@...cle.com, Liu Shixin <liushixin2@...wei.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>,
 Kenneth W Chen <kenneth.w.chen@...el.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Nanyong Sun <sunnanyong@...wei.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: hugetlb: independent PMD page table shared count

On 16.12.24 19:33, jane.chu@...cle.com wrote:
> On 12/14/2024 2:44 AM, Liu Shixin wrote:
> 
>> The folio refcount may be increased unexpectly through try_get_folio() by
>> caller such as split_huge_pages. In huge_pmd_unshare(), we use refcount to
>> check whether a pmd page table is shared. The check is incorrect if the
>> refcount is increased by the above caller, and this can cause the page
>> table leaked:
> 
> hugetlb and THP don't overlap, right?  how does split_huge_pages() end
> up messing up huge_pmd_share() ?
> 
> Am I missing something?
> 

If first grabs a reference to then check if it's a THP. So we can end up 
grabbing anything temporarily.


In general, we'll have to be smarter about what we try grabbing, but 
handling races for now is tricky.

-- 
Cheers,

David / dhildenb