On Wed, Aug 14, 2019 at 03:49:29PM -0700, Andrew Morton wrote: > On Tue, 13 Aug 2019 16:31:35 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> wrote: > > > > > Move the debug checks to after verifying PageBuddy is true. This isolates > > > > the scope of the checks to only be for buddy pages which are on the zone's > > > > freelist which move_freepages_block() is operating on. In this case, an > > > > incorrect node or zone is a bug worthy of being warned about (and the > > > > examination of struct page is acceptable bcause this memory is not > > > > reserved). > > > > > > I'm thinking Fixes:907ec5fca3dc and Cc:stable? But 907ec5fca3dc is > > > almost a year old, so you were doing something special to trigger this? > > > > > > > We noticed it almost immediately after bringing 907ec5fca3dc in on > > CONFIG_DEBUG_VM builds. It depends on finding specific free pages in the > > per-zone free area where the math in move_freepages() will bring the start > > or end pfn into reserved memory and wanting to claim that entire pageblock > > as a new migratetype. So the path will be rare, require CONFIG_DEBUG_VM, > > and require fallback to a different migratetype. > > > > Some struct pages were already zeroed from reserve pages before > > 907ec5fca3c so it theoretically could trigger before this commit. I think > > it's rare enough under a config option that most people don't run that > > others may not have noticed. I wouldn't argue against a stable tag and > > the backport should be easy enough, but probably wouldn't single out a > > commit that this is fixing. > > OK, thanks. I added the above two paragraphs to the changelog and > removed the Fixes: > > Hopefully Mel will be able to review this for us. Bit late as I was offline but FWIW Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> That said, the overhead of the debugging check is higher with this patch although it'll only affect debug builds and the path is not particularly hot. If this was a concern, I think it would be reasonable to simply remove the debugging check as the zone boundaries are checked in move_freepages_block and we never expect a zone/node to be smaller than a pageblock and stuck in the middle of another zone. -- Mel Gorman SUSE Labs
