Re: [PATCH] proc/pagemap: walk page tables under pte lock

[Cyrill Gorcunov]

On Mon, Jan 26, 2015 at 05:52:14PM +0300, Konstantin Khlebnikov wrote:
> Lockless access to pte in pagemap_pte_range() might race with page migration
> and trigger BUG_ON(!PageLocked()) in migration_entry_to_page():
> 
> CPU A (pagemap)                           CPU B (migration)
>                                           lock_page()
>                                           try_to_unmap(page, TTU_MIGRATION...)
>                                                make_migration_entry()
>                                                set_pte_at()
> <read *pte>
> pte_to_pagemap_entry()
>                                           remove_migration_ptes()
>                                           unlock_page()
>     if(is_migration_entry())
>         migration_entry_to_page()
>             BUG_ON(!PageLocked(page))
> 
> Also lockless read might be non-atomic if pte is larger than wordsize.
> Other pte walkers (smaps, numa_maps, clear_refs) already lock ptes.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Reported-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
> Fixes: 052fb0d635df ("proc: report file/anon bit in /proc/pid/pagemap")
> Cc: Stable <stable@xxxxxxxxxxxxxxx> (v3.5+)
Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>

Thank you!
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