On Wed, Nov 25, 2020 at 12:59:58PM -0500, Andrea Arcangeli wrote: > On Wed, Nov 25, 2020 at 10:30:53AM +0000, Mel Gorman wrote: > > On Tue, Nov 24, 2020 at 03:56:22PM -0500, Andrea Arcangeli wrote: > > > Hello, > > > > > > On Tue, Nov 24, 2020 at 01:32:05PM +0000, Mel Gorman wrote: > > > > I would hope that is not the case because they are not meant to overlap. > > > > However, if the beginning of the pageblock was not the start of a zone > > > > then the pages would be valid but the pfn would still be outside the > > > > zone boundary. If it was reserved, the struct page is valid but not > > > > suitable for set_pfnblock_flags_mask. However, it is a concern in > > > > general because the potential is there that pages are isolated from the > > > > wrong zone. > > > > > > I guess we have more than one issue to correct in that function > > > because the same BUG_ON reproduced again even with the tentative patch > > > I posted earlier. > > > > > > So my guess is that the problematic reserved page isn't pointed by the > > > min_pfn, but it must have been pointed by the "highest" variable > > > calculated below? > > > > > > if (pfn >= highest) > > > highest = pageblock_start_pfn(pfn); > > > > > > When I looked at where "highest" comes from, it lacks > > > pageblock_pfn_to_page check (which was added around v5.7 to min_pfn). > > > > > > Is that the real bug, which may be fixed by something like this? (untested) > > > > > > > It's plausible as it is a potential source of leaking but as you note > > in another mail, it's surprising to me that valid struct pages, even if > > within memory holes and reserved would have broken node/zone information > > in the page flags. > > I think the patch to add pageblock_pfn_to_page is still needed to cope > with !pfn_valid or a pageblock in between zones, but pfn_valid or > pageblock in between zones is not what happens here. > > So the patch adding pageblock_pfn_to_page would have had the undesired > side effect of hiding the bug so it's best to deal with the other bug > first. > Agreed. This thread has a lot of different directions in it at this point so what I'd hope for is first, a patch that initialises holes with zone/node linkages within a 1<<(MAX_ORDER-1) alignment. If there is a hole, it would be expected the pages are PageReserved. Second, a fix to fast_isolate that forces PFNs returned to always be within the stated zone boundaries. The first is because there are assumptions that without HOLES_IN_ZONE, a true pfn_valid within 1<<(MAX_ORDER-1) means pfn_valid would be true for any PFN within that range. That assumption is relaxed in many cases -- e.g. the page allocator may not care at the moment because of how it orders checks but compaction assumes that pfn_valid within a pageblock means that all PFNs within that pageblock are valid. -- Mel Gorman SUSE Labs
