linux-kernel - Re: [PATCH] mm/gup: stop leaking pinned pages in low memory conditions

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Message-ID: <a84d415d-dd18-49ef-b72a-ee381a44a429@nvidia.com>
Date: Thu, 17 Oct 2024 14:57:05 -0700
From: John Hubbard <jhubbard@...dia.com>
To: David Hildenbrand <david@...hat.com>, Alistair Popple <apopple@...dia.com>
CC: Andrew Morton <akpm@...ux-foundation.org>, LKML
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, Shigeru Yoshida
	<syoshida@...hat.com>, Jason Gunthorpe <jgg@...dia.com>, Minchan Kim
	<minchan@...nel.org>, Pasha Tatashin <pasha.tatashin@...een.com>
Subject: Re: [PATCH] mm/gup: stop leaking pinned pages in low memory
 conditions

On 10/17/24 2:47 PM, David Hildenbrand wrote:
> On 17.10.24 23:28, Alistair Popple wrote:
>> David Hildenbrand <david@...hat.com> writes:
>>> On 16.10.24 22:22, John Hubbard wrote:
...
>>>> +        if (rc != -EAGAIN && rc != 0)
>>>> +            unpin_user_pages(pages, nr_pinned_pages);
>>>> +
>>>>        } while (rc == -EAGAIN);
>>>
>>> Wouldn't it be cleaner to simply have here after the loop (possibly
>>> even after the memalloc_pin_restore())
>>>
>>> if (rc)
>>>     unpin_user_pages(pages, nr_pinned_pages);
>>>
>>> But maybe I am missing something.
>>
>> I initially thought the same thing but I'm not sure it is
>> correct. Consider what happens when __get_user_pages_locked() fails
>> earlier in the loop. In this case it will have bailed out of the loop
>> with rc <= 0 but we shouldn't call unpin_user_pages().

doh. yes. Thanks for catching that, Alistair! I actually considered
it during the first draft, too, but got tunnel vision during the
review, sigh.

> 
> Ah, I see what you mean, I primarily only stared at the diff.
> 
> We should likely avoid using nr_pinned_pages as a temporary variable that
> can hold an error value.
> 

OK, I still want to defer all the pretty refactoring ideas into some
future effort, so for now, let's just leave v1 alone except for fixing
the typo in the comment, yes?

I'll still send out a 2-patch series with that, plus a suitably
modified fix for the memfd case too.


thanks,
-- 
John Hubbard