Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> writes: > On Mon, Aug 1, 2022 at 8:32 PM Alistair Popple <apopple@xxxxxxxxxx> wrote: >> >> When pinning pages with FOLL_LONGTERM check_and_migrate_movable_pages() >> is called to migrate pages out of zones which should not contain any >> longterm pinned pages. >> >> When migration succeeds all pages will have been unpinned so pinning >> needs to be retried. This is indicated by returning zero. When all pages >> are in the correct zone the number of pinned pages is returned. >> >> However migration can also fail, in which case pages are unpinned and >> -ENOMEM is returned. However if the failure was due to not being unable >> to isolate a page zero is returned. This leads to indefinite looping in >> __gup_longterm_locked(). > > Hi Alistair, > > During prohibiting pinning movable zone development, there was a > discussion where we figured that isolation errors should be transient > [1]. What isolation errors are you seeing that lead to infinite loop? > Why do they happen? Thanks for the pointer Pasha. There were reports of qemu running into the same zero page problem you reported there, see https://lore.kernel.org/linux-mm/165490039431.944052.12458624139225785964.stgit@omen/ This doesn't directly fix that problem as we need to allow pinning of the zero page, but it does prevent the infinite loop. I was going to re-spin this patch to retry instead of instant failure however reading that thread it seems the infinite loop is desired behaviour. So will re-spin this to leave that in-place. - Alistair > Pasha > > [1] https://lore.kernel.org/linux-mm/20201218104655.GW32193@xxxxxxxxxxxxxx
