On Tue, 27 Apr 2021, Axel Rasmussen wrote: > In a previous commit, we added the mcopy_atomic_install_pte() helper. > This helper does the job of setting up PTEs for an existing page, to map > it into a given VMA. It deals with both the anon and shmem cases, as > well as the shared and private cases. > > In other words, shmem_mcopy_atomic_pte() duplicates a case it already > handles. So, expose it, and let shmem_mcopy_atomic_pte() use it > directly, to reduce code duplication. > > This requires that we refactor shmem_mcopy_atomic_pte() a bit: > > Instead of doing accounting (shmem_recalc_inode() et al) part-way > through the PTE setup, do it afterward. This frees up > mcopy_atomic_install_pte() from having to care about this accounting, > and means we don't need to e.g. shmem_uncharge() in the error path. > > A side effect is this switches shmem_mcopy_atomic_pte() to use > lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add(). > This wrapper does some extra accounting in an exceptional case, if > appropriate, so it's actually the more correct thing to use. > > Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> Not quite. Two things. One, in this version, delete_from_page_cache(page) has vanished from the particular error path which needs it. Two, and I think this predates your changes (so needs a separate fix patch first, for backport to stable? a user with bad intentions might be able to trigger the BUG), in pondering the new error paths and that /* don't free the page */ one in particular, isn't it the case that the shmem_inode_acct_block() on entry might succeed the first time, but atomic copy fail so -ENOENT, then something else fill up the tmpfs before the retry comes in, so that retry then fail with -ENOMEM, and hit the BUG_ON(page) in __mcopy_atomic()? (As I understand it, the shmem_inode_unacct_blocks() has to be done before returning, because the caller may be unable to retry.) What the right fix is rather depends on other uses of __mcopy_atomic(): if they obviously cannot hit that BUG_ON(page), you may prefer to leave it in, and fix it here where shmem_inode_acct_block() fails. Or you may prefer instead to delete that "else BUG_ON(page);" - looks as if that would end up doing the right thing. Peter may have a preference. (Or, we could consider doing the shmem_inode_acct_block() only after the page has been copied in: its current placing reflects how shmem.c does it elsewhere, and there's reason for that, but it doesn't always work out right. Don't be surprised if I change the ordering in future, but it's probably best not to mess with that ordering now.) Sorry, if this is a pre-existing issue, then we are taking advantage of you, in asking you to fix it: but I hope that while you're in there, it will make sense to do so. Thanks, Hugh > --- > include/linux/userfaultfd_k.h | 5 ++++ > mm/shmem.c | 48 +++++------------------------------ > mm/userfaultfd.c | 17 +++++-------- > 3 files changed, 18 insertions(+), 52 deletions(-)
