[PATCH 0/4] mm/hugetlb: Early cow on fork, and a few cleanups

Peter Xu
2021-02-03T21:11:39Z

As reported by Gal [1], we still miss the code clip to handle early cow for
hugetlb case, which is true. Again, it still feels odd to fork() after using a
few huge pages, especially if they're privately mapped to me.. However I do
agree with Gal and Jason in that we should still have that since that'll
complete the early cow on fork effort at least, and it'll still fix issues
where buffers are not well under control and not easy to apply MADV_DONTFORK.

The first two patches (1-2) are some cleanups I noticed when reading into the
hugetlb reserve map code. I think it's good to have but they're not necessary
for fixing the fork issue.

The last two patches (3-4) is the real fix.

I tested this with a fork() after some vfio-pci assignment, so I'm pretty sure
the page copy path could trigger well (page will be accounted right after the
fork()), but I didn't do data check since the card I assigned is some random
nic. Gal, please feel free to try this if you have better way to verify the
series.

https://github.com/xzpeter/linux/tree/fork-cow-pin-huge

Please review, thanks!

[1] https://lore.kernel.org/lkml/27564187-4a08-f187-5a84-3df50009f6ca@amazon.com/

Peter Xu (4):
hugetlb: Dedup the code to add a new file_region
hugetlg: Break earlier in add_reservation_in_range() when we can
mm: Introduce page_needs_cow_for_dma() for deciding whether cow
hugetlb: Do early cow when page pinned on src mm

include/linux/mm.h | 21 ++++++++
mm/huge_memory.c | 8 +--
mm/hugetlb.c | 129 ++++++++++++++++++++++++++++++++++-----------
mm/internal.h | 5 --
mm/memory.c | 7 +--
5 files changed, 123 insertions(+), 47 deletions(-)

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2.26.2