linux-kernel - Re: [PATCH v3] mm/gup: stop leaking pinned pages in low memory conditions

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Message-ID: <87ttd6atxi.fsf@nvdebian.thelocal>
Date: Mon, 21 Oct 2024 16:39:55 +1100
From: Alistair Popple <apopple@...dia.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, LKML
 <linux-kernel@...r.kernel.org>, linux-mm@...ck.org, David Hildenbrand
 <david@...hat.com>, Shigeru Yoshida <syoshida@...hat.com>, Jason Gunthorpe
 <jgg@...dia.com>, Minchan Kim <minchan@...nel.org>, Pasha Tatashin
 <pasha.tatashin@...een.com>
Subject: Re: [PATCH v3] mm/gup: stop leaking pinned pages in low memory
 conditions


John Hubbard <jhubbard@...dia.com> writes:

> On 10/20/24 4:26 PM, Alistair Popple wrote:
>> John Hubbard <jhubbard@...dia.com> writes:
>> [...]
>>> @@ -2437,8 +2440,10 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
>>>   	long i, ret;
>>>     	folios = kmalloc_array(nr_pages, sizeof(*folios), GFP_KERNEL);
>>> -	if (!folios)
>>> +	if (!folios) {
>>> +		unpin_user_pages(pages, nr_pages);
>> ie. Doesn't this unpinning need to happen in
>> check_and_migrate_movable_folios()?
>
> It already does.
>
> check_and_migrate_movable_folios() calls
> migrate_longterm_unpinnable_folios(), which unpins if errors occur.

Right you are.

Reviewed-by: Alistair Popple <apopple@...dia.com>

As an aside for future clean-ups we could probably get something nicer
if we reversed the process of pin/migrate to migrate/pin. In other words
if FOLL_LONGERM try and migrate the entire range first out of
ZONE_MOVABLE first. Migration invovles walking page tables and getting a
reference on the pages anyway, so if it turns out there is nothing to
migrate you haven't lost anything performance wise.

> thanks,