reply: [RFC] pin_user_pages_fast failure count increased

["黄朝阳 (Zhaoyang Huang)" <zhaoyang.huang@unisoc.com>]

>On Mon, Apr 28, 2025 at 02:12:57PM -0700, John Hubbard wrote:
>> On 4/28/25 1:56 PM, David Hildenbrand wrote:
>> > On 28.04.25 22:14, John Hubbard wrote:
>> > > On 4/28/25 8:17 AM, Jaewon Kim wrote:
>> > > > Hi
>> > > >
>> > > > If pin_user_pages_fast does not pin all the requested number of
>> > > > pages, then drivers calling to pin_user_pages_fast should retry
>> > > > until the gup pins all?
>> > > >
>> > >
>> > > Approaches vary, for handling partial success of pin_user_pages().
>> > >
>> > > * Many drivers unpin everything and either bail out entirely, or
>> > > retry pinning the entire original range.
>> >
>> > Hm, unpinning + trying to repin the entire range can easily result
>> > in an endless loop on persistent errors IIRC?
>> >
>>
>> I vaguely recall a limited number of retries, yes.
>>
>> thanks,
>> --
>> John Hubbard
>>
>>
>
>Hi,
>
>I'd like to report a potential issue introduced by a recent change in
>1aaf8c122918 mm: gup: fix infinite loop within __get_longterm_locked
>
>Previously, the call to migrate_longterm_unpinnable_folio() was guarded by the
>collected variable. This meant that if a CMA page was temporarily held in the
>pagevec and failed LRU isolation, it wouldn't be added to the
>movable_page_list, but the collected counter would still be incremented.
There is lru_add_drain_all for dealing with this scenario, so this won't be the case, right?
>
>As a result, migrate_longterm_unpinnable_folio() would return -EAGAIN, and
>the process would be retried until migration of the CMA page succeeded.
>
>However, in the recent patch merged into mainline, the logic now only checks
>whether movable_page_list is empty, and no longer relies on the collected
>count.
>This can cause CMA pages that fail isolation to bypass retry logic and remain
>pinned.
>
>Effectively,long-term pinning is now possible for CMA pages — something that
>previously would have been avoided through repeated attempts.
>
>We've observed this behavior in practice, which has led to issues such as CMA
>allocation failures under memory pressure. This may indicate a regression in
>the logic that prevents pinning of unmovable CMA pages.
>
>I believe this warrants further discussion or possibly a fix to restore the
>intended retry behavior for pages that fail LRU isolation.
>
>Thanks,
>Hyesoo Yu.