On Mon, 22 Jun 2015, Michal Hocko wrote: > On Fri 19-06-15 12:43:33, Eric B Munson wrote: > > 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. > > One could use /proc/<pid>/pagemap to query the residency. > > > This assumes that > > MAP_FAULTPOPULATE does not translate to a VMA flag, but it sounds like > > it would have to. > > Yes, it would have to have a VM flag for the vma. > > > > 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)? > > I thought the MAP_FAULTPOPULATE (or any other better name) would > directly translate into VM_FAULTPOPULATE and wouldn't be tight to the > locked semantic. We already have VM_LOCKED for that. The direct effect > of the flag would be to prevent from population other than the direct > page fault - including any speculative actions like fault around or > read-ahead. I like the ability to control other speculative population, but I am not sure about overloading it with the VM_LOCKONFAULT case. Here is my concern. If we are using VM_FAULTPOPULATE | VM_LOCKED to denote LOCKONFAULT, how can we tell the difference between someone that wants to avoid read-ahead and wants to use mlock()? This might lead to some interesting states with mlock() and munlock() that take flags. For instance, using VM_LOCKONFAULT mlock(MLOCK_ONFAULT) followed by munlock(MLOCK_LOCKED) leaves the VMAs in the same state with VM_LOCKONFAULT set. If we use VM_FAULTPOPULATE, the same pair of calls would clear VM_LOCKED, but leave VM_FAULTPOPULATE. It may not matter in the end, but I am concerned about the subtleties here. > > > 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? > > OK I see your point now. We would indeed need a flag argument for mlock. > -- > Michal Hocko > SUSE Labs
Attachment:
signature.asc
Description: Digital signature
