On Fri, 19 Jun 2015, Michal Hocko wrote: > On Thu 18-06-15 16:30:48, Eric B Munson wrote: > > On Thu, 18 Jun 2015, Michal Hocko wrote: > [...] > > > Wouldn't it be much more reasonable and straightforward to have > > > MAP_FAULTPOPULATE as a counterpart for MAP_POPULATE which would > > > explicitly disallow any form of pre-faulting? It would be usable for > > > other usecases than with MAP_LOCKED combination. > > > > I don't see a clear case for it being more reasonable, it is one > > possible way to solve the problem. > > MAP_FAULTPOPULATE would be usable for other cases as well. E.g. fault > around is all or nothing feature. Either all mappings (which support > this) fault around or none. There is no way to tell the kernel that > this particular mapping shouldn't fault around. I haven't seen such a > request yet but we have seen requests to have a way to opt out from > a global policy in the past (e.g. per-process opt out from THP). So > I can imagine somebody will come with a request to opt out from any > speculative operations on the mapped area in the future. > > > But I think it leaves us in an even > > more akward state WRT VMA flags. As you noted in your fix for the > > mmap() man page, one can get into a state where a VMA is VM_LOCKED, but > > not present. Having VM_LOCKONFAULT states that this was intentional, if > > we go to using MAP_FAULTPOPULATE instead of MAP_LOCKONFAULT, we no > > longer set VM_LOCKONFAULT (unless we want to start mapping it to the > > presence of two MAP_ flags). This can make detecting the MAP_LOCKED + > > populate failure state harder. > > I am not sure I understand your point here. Could you be more specific > how would you check for that and what for? My thought on detecting was that someone might want to know if they had a VMA that was VM_LOCKED but had not been made present becuase of a failure in mmap. We don't have a way today, but adding VM_LOCKONFAULT is at least explicit about what is happening which would make detecting the VM_LOCKED but not present state easier. This assumes that MAP_FAULTPOPULATE does not translate to a VMA flag, but it sounds like it would have to. > > From my understanding MAP_LOCKONFAULT is essentially > MAP_FAULTPOPULATE|MAP_LOCKED with a quite obvious semantic (unlike > single MAP_LOCKED unfortunately). I would love to also have > MAP_LOCKED|MAP_POPULATE (aka full mlock semantic) but I am really > skeptical considering how my previous attempt to make MAP_POPULATE > reasonable went. Are you objecting to the addition of the VMA flag VM_LOCKONFAULT, or the new MAP_LOCKONFAULT flag (or both)? If you prefer that MAP_LOCKED | MAP_FAULTPOPULATE means that VM_LOCKONFAULT is set, I am fine with that instead of introducing MAP_LOCKONFAULT. I went with the new flag because to date, we have a one to one mapping of MAP_* to VM_* flags. > > > If this is the preferred path for mmap(), I am fine with that. > > > However, > > I would like to see the new system calls that Andrew mentioned (and that > > I am testing patches for) go in as well. > > mlock with flags sounds like a good step but I am not sure it will make > sense in the future. POSIX has screwed that and I am not sure how many > applications would use it. This ship has sailed long time ago. I don't know either, but the code is the question, right? I know that we have at least one team that wants it here. > > > That way we give users the > > ability to request VM_LOCKONFAULT for memory allocated using something > > other than mmap. > > mmap(MAP_FAULTPOPULATE); mlock() would have the same semantic even > without changing mlock syscall. That is true as long as MAP_FAULTPOPULATE set a flag in the VMA(s). It doesn't cover the actual case I was asking about, which is how do I get lock on fault on malloc'd memory? > > > > > This patch introduces the ability to request that pages are not > > > > pre-faulted, but are placed on the unevictable LRU when they are finally > > > > faulted in. > > > > > > > > To keep accounting checks out of the page fault path, users are billed > > > > for the entire mapping lock as if MAP_LOCKED was used. > > > > > > > > Signed-off-by: Eric B Munson <emunson@xxxxxxxxxx> > > > > Cc: Michal Hocko <mhocko@xxxxxxx> > > > > Cc: linux-alpha@xxxxxxxxxxxxxxx > > > > Cc: linux-kernel@xxxxxxxxxxxxxxx > > > > Cc: linux-mips@xxxxxxxxxxxxxx > > > > Cc: linux-parisc@xxxxxxxxxxxxxxx > > > > Cc: linuxppc-dev@xxxxxxxxxxxxxxxx > > > > Cc: sparclinux@xxxxxxxxxxxxxxx > > > > Cc: linux-xtensa@xxxxxxxxxxxxxxxx > > > > Cc: linux-mm@xxxxxxxxx > > > > Cc: linux-arch@xxxxxxxxxxxxxxx > > > > Cc: linux-api@xxxxxxxxxxxxxxx > > > > --- > [...] > -- > Michal Hocko > SUSE Labs
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