Messages in this thread | | | Date | Tue, 23 Jun 2015 15:04:22 +0200 | From | Vlastimil Babka <> | Subject | Re: [RESEND PATCH V2 0/3] Allow user to request memory to be locked on page fault |
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On 06/15/2015 04:43 PM, Eric B Munson wrote: >> Note that the semantic of MAP_LOCKED can be subtly surprising: >> >> "mlock(2) fails if the memory range cannot get populated to guarantee >> that no future major faults will happen on the range. >> mmap(MAP_LOCKED) on the other hand silently succeeds even if the >> range was populated only >> partially." >> >> ( from http://marc.info/?l=linux-mm&m=143152790412727&w=2 ) >> >> So MAP_LOCKED can silently behave like MAP_LOCKONFAULT. While >> MAP_LOCKONFAULT doesn't suffer from such problem, I wonder if that's >> sufficient reason not to extend mmap by new mlock() flags that can >> be instead applied to the VMA after mmapping, using the proposed >> mlock2() with flags. So I think instead we could deprecate >> MAP_LOCKED more prominently. I doubt the overhead of calling the >> extra syscall matters here? > > We could talk about retiring the MAP_LOCKED flag but I suspect that > would get significantly more pushback than adding a new mmap flag.
Oh no we can't "retire" as in remove the flag, ever. Just not continue the way of mmap() flags related to mlock().
> Likely that the overhead does not matter in most cases, but presumably > there are cases where it does (as we have a MAP_LOCKED flag today). > Even with the proposed new system calls I think we should have the > MAP_LOCKONFAULT for parity with MAP_LOCKED.
I'm not convinced, but it's not a major issue.
>> >>> - mlock() takes a `flags' argument. Presently that's >>> MLOCK_LOCKED|MLOCK_LOCKONFAULT. >>> >>> - munlock() takes a `flags' arument. MLOCK_LOCKED|MLOCK_LOCKONFAULT >>> to specify which flags are being cleared. >>> >>> - mlockall() and munlockall() ditto. >>> >>> >>> IOW, LOCKED and LOCKEDONFAULT are treated identically and independently. >>> >>> Now, that's how we would have designed all this on day one. And I >>> think we can do this now, by adding new mlock2() and munlock2() >>> syscalls. And we may as well deprecate the old mlock() and munlock(), >>> not that this matters much. >>> >>> *should* we do this? I'm thinking "yes" - it's all pretty simple >>> boilerplate and wrappers and such, and it gets the interface correct, >>> and extensible. >> >> If the new LOCKONFAULT functionality is indeed desired (I haven't >> still decided myself) then I agree that would be the cleanest way. > > Do you disagree with the use cases I have listed or do you think there > is a better way of addressing those cases?
I'm somewhat sceptical about the security one. Are security sensitive buffers that large to matter? The performance one is more convincing and I don't see a better way, so OK.
> >> >>> What do others think?
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