Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [RFC PATCH 0/5] madvise MADV_DOEXEC | Date | Mon, 16 Aug 2021 17:01:44 +0200 |
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On 16.08.21 16:40, Matthew Wilcox wrote: > On Mon, Aug 16, 2021 at 04:33:09PM +0200, David Hildenbrand wrote: >>>> I did not follow why we have to play games with MAP_PRIVATE, and having >>>> private anonymous pages shared between processes that don't COW, introducing >>>> new syscalls etc. >>> >>> It's not about SHMEM, it's about file-backed pages on regular >>> filesystems. I don't want to have XFS, ext4 and btrfs all with their >>> own implementations of ARCH_WANT_HUGE_PMD_SHARE. >> >> Let me ask this way: why do we have to play such games with MAP_PRIVATE? > > Are you referring to this?
Yes
> > : Mappings within this address range behave as if they were shared > : between threads, so a write to a MAP_PRIVATE mapping will create a > : page which is shared between all the sharers. > > If so, that's a misunderstanding, because there are no games being played. > What Khalid's saying there is that because the page tables are already > shared for that range of address space, the COW of a MAP_PRIVATE will > create a new page, but that page will be shared between all the sharers. > The second write to a MAP_PRIVATE page (by any of the sharers) will not > create a COW situation. Just like if all the sharers were threads of > the same process. >
It actually seems to be just like I understood it. We'll have multiple processes share anonymous pages writable, even though they are not using shared memory.
IMHO, sharing page tables to optimize for something kernel-internal (page table consumption) should be completely transparent to user space. Just like ARCH_WANT_HUGE_PMD_SHARE currently is unless I am missing something important.
The VM_MAYSHARE check in want_pmd_share()->vma_shareable() makes me assume that we really only optimize for MAP_SHARED right now, never for MAP_PRIVATE.
-- Thanks,
David / dhildenb
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