Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory | Date | Thu, 18 Feb 2021 12:38:27 +0100 |
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>>>> If we hit >>>> hardware errors on pages, ignore them - nothing we really can or >>>> should do. >>>> 3. On errors during MADV_POPULATED, some memory might have been >>>> populated. Callers have to clean up if they care. >>> >>> How does caller find out? madvise reports 0 on success so how do you >>> find out how much has been populated? >> >> If there is an error, something might have been populated. In my QEMU >> implementation, I simply discard the range again, good enough. I don't think >> we need to really indicate "error and populated" or "error and not >> populated". > > Agreed. The wording just suggests that the syscall actually provides any > means for an effective way to handle those errors. Maybe you should just > stick with the first sentence and drop the second.
Makes sense. "On errors during MADV_POPULATE, some memory might have been populated."
> >>>> 4. Concurrent changes to the virtual memory layour are tolerated - we >>>> process each and every PFN only once, though. >>> >>> I do not understand this. madvise is about virtual address space not a >>> physical address space. >> >> What I wanted to express: if we detect a change in the mapping we don't >> restart at the beginning, we always make forward progress. We process each >> virtual address once (on a per-page basis, thus I accidentally used "PFN"). > > This is an implicit assumption. Your range can have the same page mapped > several times in the given address range and all you care about is that > you fault those which are not present during the virtual address space > walk. Your syscall can return and large part of the address space might > be unpopulated because memory reclaim just dropped those pages and that > would be fine. This shouldn't really imply memory presence - mlock does > that.
"Concurrent changes to the virtual memory layout are tolerated. The range is processed exactly once."
> >>>> 5. If MADV_POPULATE succeeds, all memory in the range can be accessed >>>> without SIGBUS. (of course, not if user space changed mappings in the >>>> meantime or KSM kicked in on anonymous memory). >>> >>> I do not see how KSM would change anything here and maybe it is not >>> really important to mention it. KSM should be really transparent from >>> the users space POV. Parallel and destructive virtual address space >>> operations are also expected to change the outcome and there is nothing >>> kernel do about at and provide any meaningful guarantees. I guess we >>> want to assume a reasonable userspace behavior here. >> >> It's just a note that we cannot protect from someone interfering >> (discard/ksm/whatever). I'm making that clearer in the cover letter. > > Again that is implicit expectation. madvise will not work for anybody > shooting an own foot.
Okay, I'll drop that part, thanks!
-- Thanks,
David / dhildenb
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