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Date: Thu, 28 Jan 2021 21:08:35 +0800 From: Muchun Song <songmuchun@...edance.com> To: David Hildenbrand <david@...hat.com>, Oscar Salvador <osalvador@...e.de>, Mike Kravetz <mike.kravetz@...cle.com> Cc: Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org, Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk, Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org, mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com, Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de, Mina Almasry <almasrymina@...gle.com>, David Rientjes <rientjes@...gle.com>, Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>, "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>, HORIGUCHI NAOYA(堀口 直也) <naoya.horiguchi@....com>, Xiongchun duan <duanxiongchun@...edance.com>, linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, Linux Memory Management List <linux-mm@...ck.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org> Subject: Re: [External] Re: [PATCH v13 05/12] mm: hugetlb: allocate the vmemmap pages associated with each HugeTLB page On Thu, Jan 28, 2021 at 8:37 PM Muchun Song <songmuchun@...edance.com> wrote: > > On Wed, Jan 27, 2021 at 6:36 PM David Hildenbrand <david@...hat.com> wrote: > > > > On 26.01.21 16:56, David Hildenbrand wrote: > > > On 26.01.21 16:34, Oscar Salvador wrote: > > >> On Tue, Jan 26, 2021 at 04:10:53PM +0100, David Hildenbrand wrote: > > >>> The real issue seems to be discarding the vmemmap on any memory that has > > >>> movability constraints - CMA and ZONE_MOVABLE; otherwise, as discussed, we > > >>> can reuse parts of the thingy we're freeing for the vmemmap. Not that it > > >>> would be ideal: that once-a-huge-page thing will never ever be a huge page > > >>> again - but if it helps with OOM in corner cases, sure. > > >> > > >> Yes, that is one way, but I am not sure how hard would it be to implement. > > >> Plus the fact that as you pointed out, once that memory is used for vmemmap > > >> array, we cannot use it again. > > >> Actually, we would fragment the memory eventually? > > >> > > >>> Possible simplification: don't perform the optimization for now with free > > >>> huge pages residing on ZONE_MOVABLE or CMA. Certainly not perfect: what > > >>> happens when migrating a huge page from ZONE_NORMAL to (ZONE_MOVABLE|CMA)? > > >> > > >> But if we do not allow theose pages to be in ZONE_MOVABLE or CMA, there is no > > >> point in migrate them, right? > > > > > > Well, memory unplug "could" still work and migrate them and > > > alloc_contig_range() "could in the future" still want to migrate them > > > (virtio-mem, gigantic pages, powernv memtrace). Especially, the latter > > > two don't work with ZONE_MOVABLE/CMA. But, I mean, it would be fair > > > enough to say "there are no guarantees for > > > alloc_contig_range()/offline_pages() with ZONE_NORMAL, so we can break > > > these use cases when a magic switch is flipped and make these pages > > > non-migratable anymore". > > > > > > I assume compaction doesn't care about huge pages either way, not sure > > > about numa balancing etc. > > > > > > > > > However, note that there is a fundamental issue with any approach that > > > allocates a significant amount of unmovable memory for user-space > > > purposes (excluding CMA allocations for unmovable stuff, CMA is > > > special): pairing it with ZONE_MOVABLE becomes very tricky as your user > > > space might just end up eating all kernel memory, although the system > > > still looks like there is plenty of free memory residing in > > > ZONE_MOVABLE. I mentioned that in the context of secretmem in a reduced > > > form as well. > > > > > > We theoretically have that issue with dynamic allocation of gigantic > > > pages, but it's something a user explicitly/rarely triggers and it can > > > be documented to cause problems well enough. We'll have the same issue > > > with GUP+ZONE_MOVABLE that Pavel is fixing right now - but GUP is > > > already known to be broken in various ways and that it has to be treated > > > in a special way. I'd like to limit the nasty corner cases. > > > > > > Of course, we could have smart rules like "don't online memory to > > > ZONE_MOVABLE automatically when the magic switch is active". That's just > > > ugly, but could work. > > > > > > > Extending on that, I just discovered that only x86-64, ppc64, and arm64 > > really support hugepage migration. > > > > Maybe one approach with the "magic switch" really would be to disable > > hugepage migration completely in hugepage_migration_supported(), and > > consequently making hugepage_movable_supported() always return false. > > > > Huge pages would never get placed onto ZONE_MOVABLE/CMA and cannot be > > migrated. The problem I describe would apply (careful with using > > ZONE_MOVABLE), but well, it can at least be documented. > > Thanks for your explanation. > > All thinking seems to be introduced by encountering OOM. :-( > > In order to move forward and free the hugepage. We should add some > restrictions below. > > 1. Only free the hugepage which is allocated from the ZONE_NORMAL. ^^ Sorry. Here "free" should be "optimize". > 2. Disable hugepage migration when this feature is enabled. > 3. Using GFP_ATOMIC to allocate vmemmap pages firstly (it can reduce > memory fragmentation), if it fails, we use part of the hugepage to > remap. > > Hi Oscar, Mike and David H > > What's your opinion about this? Should we take this approach? > > Thanks. > > > > > -- > > Thanks, > > > > David / dhildenb > >
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