On 11/12/24 11:11 AM, Brian Foster wrote: > On Tue, Nov 12, 2024 at 10:13:12AM -0700, Jens Axboe wrote: >> On 11/12/24 9:36 AM, Brian Foster wrote: >>> On Mon, Nov 11, 2024 at 04:37:39PM -0700, Jens Axboe wrote: >>>> IOCB_UNCACHED IO needs to prune writeback regions on IO completion, >>>> and hence need the worker punt that ext4 also does for unwritten >>>> extents. Add an io_end flag to manage that. >>>> >>>> If foliop is set to foliop_uncached in ext4_write_begin(), then set >>>> FGP_UNCACHED so that __filemap_get_folio() will mark newly created >>>> folios as uncached. That in turn will make writeback completion drop >>>> these ranges from the page cache. >>>> >>>> Now that ext4 supports both uncached reads and writes, add the fop_flag >>>> FOP_UNCACHED to enable it. >>>> >>>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> >>>> --- >>>> fs/ext4/ext4.h | 1 + >>>> fs/ext4/file.c | 2 +- >>>> fs/ext4/inline.c | 7 ++++++- >>>> fs/ext4/inode.c | 18 ++++++++++++++++-- >>>> fs/ext4/page-io.c | 28 ++++++++++++++++------------ >>>> 5 files changed, 40 insertions(+), 16 deletions(-) >>>> >>> ... >>>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c >>>> index 54bdd4884fe6..afae3ab64c9e 100644 >>>> --- a/fs/ext4/inode.c >>>> +++ b/fs/ext4/inode.c >>>> @@ -1138,6 +1138,7 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping, >>>> int ret, needed_blocks; >>>> handle_t *handle; >>>> int retries = 0; >>>> + fgf_t fgp_flags; >>>> struct folio *folio; >>>> pgoff_t index; >>>> unsigned from, to; >>>> @@ -1164,6 +1165,15 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping, >>>> return 0; >>>> } >>>> >>>> + /* >>>> + * Set FGP_WRITEBEGIN, and FGP_UNCACHED if foliop contains >>>> + * foliop_uncached. That's how generic_perform_write() informs us >>>> + * that this is an uncached write. >>>> + */ >>>> + fgp_flags = FGP_WRITEBEGIN; >>>> + if (*foliop == foliop_uncached) >>>> + fgp_flags |= FGP_UNCACHED; >>>> + >>>> /* >>>> * __filemap_get_folio() can take a long time if the >>>> * system is thrashing due to memory pressure, or if the folio >>>> @@ -1172,7 +1182,7 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping, >>>> * the folio (if needed) without using GFP_NOFS. >>>> */ >>>> retry_grab: >>>> - folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN, >>>> + folio = __filemap_get_folio(mapping, index, fgp_flags, >>>> mapping_gfp_mask(mapping)); >>>> if (IS_ERR(folio)) >>>> return PTR_ERR(folio); >>> >>> JFYI, I notice that ext4 cycles the folio lock here in this path and >>> thus follows up with a couple checks presumably to accommodate that. One >>> is whether i_mapping has changed, which I assume means uncached state >>> would have been handled/cleared externally somewhere..? I.e., if an >>> uncached folio is somehow truncated/freed without ever having been >>> written back? >>> >>> The next is a folio_wait_stable() call "in case writeback began ..." >>> It's not immediately clear to me if that is possible here, but taking >>> that at face value, is it an issue if we were to create an uncached >>> folio, drop the folio lock, then have some other task dirty and >>> writeback the folio (due to a sync write or something), then have >>> writeback completion invalidate the folio before we relock it here? >> >> I don't either of those are an issue. The UNCACHED flag will only be set >> on a newly created folio, it does not get inherited for folios that >> already exist. >> > > Right.. but what I was wondering for that latter case is if the folio is > created here by ext4, so uncached is set before it is unlocked. > > On second look I guess the uncached completion invalidation should clear > mapping and thus trigger the retry logic here. That seems reasonable > enough, but is it still possible to race with writeback? > > Maybe this is a better way to ask.. what happens if a write completes to > an uncached folio that is already under writeback? For example, uncached > write 1 completes, submits for writeback and returns to userspace. Then > write 2 begins and redirties the same folio before the uncached > writeback completes. > > If I follow correctly, if write 2 is also uncached, it eventually blocks > in writeback submission (folio_prepare_writeback() -> > folio_wait_writeback()). It looks like folio lock is held there, so > presumably that would bypass the completion time invalidation in > folio_end_uncached(). But what if write 2 was not uncached or perhaps > writeback completion won the race for folio lock vs. the write side > (between locking the folio for dirtying and later for writeback > submission)? Does anything prevent invalidation of the folio before the > second write is submitted for writeback? > > IOW, I'm wondering if the uncached completion time invalidation also > needs a folio dirty check..? Ah ok, I see what you mean. If the folio is dirty, the unmapping will fail. But I guess with the recent change, we'll actually unmap it first. I'll add the folio dirty check, thanks! -- Jens Axboe
