On Tue, Apr 27, 2021 at 05:58:16PM -0700, Hugh Dickins wrote: > 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. Agreed. I also spotted that the set_page_dirty() seems to have been overlooked when reusing mcopy_atomic_install_pte(), which afaiu should be move into the helper. > > 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. To me, the BUG_ON(page) wanted to guarantee mfill_atomic_pte() should have consumed the page properly when possible. Removing the BUG_ON() looks good already, it will just stop covering the case when e.g. ret==0. So maybe slightly better to release the page when shmem_inode_acct_block() fails (so as to still keep some guard on the page)? Thanks, -- Peter Xu
