linux-kernel - Re: [PATCH v3 1/7] mm: Remove BUG_ON mmap_sem not held from xxx_trans_huge

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Date:   Thu, 3 Oct 2019 13:32:09 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org,
        Thomas Hellstrom <thellstrom@...are.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Huang Ying <ying.huang@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH v3 1/7] mm: Remove BUG_ON mmap_sem not held from
 xxx_trans_huge_lock()

Hi, Kirill,

On 10/3/19 1:02 PM, Kirill A. Shutemov wrote:
> On Wed, Oct 02, 2019 at 03:47:24PM +0200, Thomas Hellström (VMware) wrote:
>> From: Thomas Hellstrom <thellstrom@...are.com>
>>
>> The caller needs to make sure that the vma is not torn down during the
>> lock operation and can also use the i_mmap_rwsem for file-backed vmas.
>> Remove the BUG_ON. We could, as an alternative, add a test that either
>> vma->vm_mm->mmap_sem or vma->vm_file->f_mapping->i_mmap_rwsem are held.
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Matthew Wilcox <willy@...radead.org>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Rik van Riel <riel@...riel.com>
>> Cc: Minchan Kim <minchan@...nel.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Huang Ying <ying.huang@...el.com>
>> Cc: Jérôme Glisse <jglisse@...hat.com>
>> Cc: Kirill A. Shutemov <kirill@...temov.name>
>> Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
> The patch looks good to me:
>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>
> But I looked at usage at pagewalk.c and it is inconsitent.  The walker
> takes ptl before calling ->pud_entry(), but not for ->pmd_entry().
>
> It should be fixed: do not take the lock before ->pud_entry(). The
> callback must take care of it.
>
> Looks like we have single ->pud_entry() implementation the whole kernel.
> It should be trivial to fix.
>
> Could you do this?
>
I could probably fix that. There are some comments in the patch 
introducing that code as to why it was done that way, though, but I 
don't remember offhand what the arguments were.

But there seems to be more races WRT puds. See my next email. Perhaps 
this should be fixed as part of a larger audit of the huge_pud code?

/Thomas