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Date:   Sun, 13 Feb 2022 00:56:14 -0800
From:   John Hubbard <jhubbard@nvidia.com>
To:     Rik van Riel <riel@surriel.com>
CC:     <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] mm: clean up hwpoison page cache page in fault path
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On Fri, 11 Feb 2022, Rik van Riel wrote:
> Sometimes the page offlining code can leave behind a hwpoisoned clean
> page cache page. This can lead to programs being killed over and over
> and over again as they fault in the hwpoisoned page, get killed, and
> then get re-spawned by whatever wanted to run them.

Hi Rik,

This looks good. Some minor suggestions below:

>     
> This is particularly embarrassing when the page was offlined due to
> having too many corrected memory errors. Now we are killing tasks
> due to them trying to access memory that probably isn't even corrupted.

I'd recommend deleting that paragraph entirely. It's a separate
question, and it is not necessarily an accurate assessment of that
question either: the engineers who set the thresholds for "too many
corrected errors" may not--in fact, probably *will not*--agree with your
feeling that the memory is still working and reliable!

>     
> This problem can be avoided by invalidating the page from the page
> fault handler, which already has a branch for dealing with these
> kinds of pages. With this patch we simply pretend the page fault
> was successful if the page was invalidated, return to userspace,
> incur another page fault, read in the file from disk (to a new
> memory page), and then everything works again.

Very nice write-up here, it's rare that I get to read such a short, yet
clear explanation. Thank you for that. :)

>     
> Signed-off-by: Rik van Riel <riel@surriel.com>
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..2300358e268c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3871,11 +3871,16 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
>  		return ret;
>  
>  	if (unlikely(PageHWPoison(vmf->page))) {
> -		if (ret & VM_FAULT_LOCKED)
> +		int poisonret = VM_FAULT_HWPOISON;
> +		if (ret & VM_FAULT_LOCKED) {

How about changing those two lines to these three (note the newline
after the declaration):

		vm_fault_t poison_ret = VM_FAULT_HWPOISON;

		if (ret & VM_FAULT_LOCKED) {

...which should fix the krobot complaint, and the underscore is just a
very tiny readability improvement while we're there.

> +			/* Retry if a clean page was removed from the cache. */

This is more cryptic than necessary, and in fact you've already provided
a write-up, so how about something like this, instead of the above:

		/*
		 * Avoid refaulting on the same poisoned page
		 * forever: attempt to invalidate the page. If that
		 * succeeds, then pretend the page fault was successful,
		 * return to userspace, incur another page fault, read
		 * in the file from disk (to a new page),and then
		 * everything works again.
		 */

> +			if (invalidate_inode_page(vmf->page))
> +				poisonret = 0;
>  			unlock_page(vmf->page);
> +		}
>  		put_page(vmf->page);
>  		vmf->page = NULL;
> -		return VM_FAULT_HWPOISON;
> +		return poisonret;
>  	}
>  
>  	if (unlikely(!(ret & VM_FAULT_LOCKED)))
> 
> 
> -- 
> All rights reversed.

Anyway, those are all documentation nits, except for the vm_fault_t,
which is a declaration nit :) , so I'm comfortable saying:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA