On Tue, 15 Mar 2022 03:32:16 +0000 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > On Tue, Mar 15, 2022 at 11:05:15AM +0800, Andrew Yang wrote: > > When memory is tight, system may start to compact memory for large > > continuous memory demands. If one process tries to lock a memory page > > that is being locked and isolated for compaction, it may wait a long time > > or even forever. This is because compaction will perform non-atomic > > PG_Isolated clear while holding page lock, this may overwrite PG_waiters > > set by the process that can't obtain the page lock and add itself to the > > waiting queue to wait for the lock to be unlocked. > > > > CPU1 CPU2 > > lock_page(page); (successful) > > lock_page(); (failed) > > __ClearPageIsolated(page); SetPageWaiters(page) (may be overwritten) > > unlock_page(page); > > > > The solution is to not perform non-atomic operation on page flags while > > holding page lock. > > > > Signed-off-by: andrew.yang <andrew.yang@xxxxxxxxxxxx> > > --- > > include/linux/page-flags.h | 2 +- > > mm/migrate.c | 12 ++++++------ > > 2 files changed, 7 insertions(+), 7 deletions(-) > > > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > > index 1c3b6e5c8bfd..64a84a9835cb 100644 > > --- a/include/linux/page-flags.h > > +++ b/include/linux/page-flags.h > > @@ -918,7 +918,7 @@ PAGE_TYPE_OPS(Guard, guard) > > > > extern bool is_free_buddy_page(struct page *page); > > > > -__PAGEFLAG(Isolated, isolated, PF_ANY); > > +PAGEFLAG(Isolated, isolated, PF_ANY); > > Agreed. Further, page cannot be a tail page (this is implied by the > get_page_unless_zero() as tailpages have a zero refcount, and it > is assumed by __PageMovable() as page->mapping is undefined for tail > pages). So this can actually be: > > +PAGEFLAG(Isolated, isolated, PF_NO_TAIL); > > I considered PF_ONLY_HEAD, but there are a lot more places that _check_ > PageIsolated() and I don't want to prove that they're all definitely > working on head pages. There was no followup to this. I'll send the patch upstream as-is, but please let's not forget this idea.
