On Wed, Apr 28, 2021 at 08:59:53AM -0700, Axel Rasmussen wrote: > On Wed, Apr 28, 2021 at 8:56 AM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > 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. > > I think this is covered: we explicitly call SetPageDirty() just before > returning in shmem_mcopy_atomic_pte(). If I remember correctly from a > couple of revisions ago, we consciously put it here instead of in the > helper because it resulted in simpler code (error handling in > particular, I think?), and not all callers of the new helper need it. Indeed, yes that looks okay. > > > > > > > > > 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)? > > This second issue, I will take some more time to investigate. :) No worry - take your time. :) -- Peter Xu
