Re: [PATCH v5 2/3] mm,hwpoison: Return -EHWPOISON to denote that the page has already been poisoned

[Oscar Salvador <osalvador@suse.de>]

On Fri, May 21, 2021 at 12:01:55PM +0900, Naoya Horiguchi wrote:
> From: Aili Yao <yaoaili@kingsoft.com>
> 
> When memory_failure() is called with MF_ACTION_REQUIRED on the
> page that has already been hwpoisoned, memory_failure() could fail
> to send SIGBUS to the affected process, which results in infinite
> loop of MCEs.
> 
> Currently memory_failure() returns 0 if it's called for already
> hwpoisoned page, then the caller, kill_me_maybe(), could return
> without sending SIGBUS to current process.  An action required MCE
> is raised when the current process accesses to the broken memory,
> so no SIGBUS means that the current process continues to run and
> access to the error page again soon, so running into MCE loop.
> 
> This issue can arise for example in the following scenarios:
> 
>   - Two or more threads access to the poisoned page concurrently.
>     If local MCE is enabled, MCE handler independently handles the
>     MCE events.  So there's a race among MCE events, and the
>     second or latter threads fall into the situation in question.
> 
>   - If there was a precedent memory error event and memory_failure()
>     for the event failed to unmap the error page for some reason,
>     the subsequent memory access to the error page triggers the
>     MCE loop situation.
> 
> To fix the issue, make memory_failure() return an error code when the
> error page has already been hwpoisoned.  This allows memory error
> handler to control how it sends signals to userspace.  And make sure
> that any process touching a hwpoisoned page should get a SIGBUS even
> in "already hwpoisoned" path of memory_failure() as is done in page
> fault path.
> 
> Signed-off-by: Aili Yao <yaoaili@kingsoft.com>
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

-- 
Oscar Salvador
SUSE L3