On Fri, Nov 15, 2024 at 08:46:13AM -0500, Brian Foster wrote: > On Fri, Nov 15, 2024 at 07:53:17PM +0800, Long Li wrote: > > On Thu, Nov 14, 2024 at 01:04:31PM -0500, Brian Foster wrote: > > > On Thu, Nov 14, 2024 at 10:34:26AM +0800, Long Li wrote: > > > > On Wed, Nov 13, 2024 at 11:13:49AM -0500, Brian Foster wrote: > > > > > FYI, you probably want to include linux-fsdevel on iomap patches. > > > > > > > > > > On Wed, Nov 13, 2024 at 05:19:06PM +0800, Long Li wrote: > > > > > > During concurrent append writes to XFS filesystem, zero padding data > > > > > > may appear in the file after power failure. This happens due to imprecise > > > > > > disk size updates when handling write completion. > > > > > > > > > > > > Consider this scenario with concurrent append writes same file: > > > > > > > > > > > > Thread 1: Thread 2: > > > > > > ------------ ----------- > > > > > > write [A, A+B] > > > > > > update inode size to A+B > > > > > > submit I/O [A, A+BS] > > > > > > write [A+B, A+B+C] > > > > > > update inode size to A+B+C > > > > > > <I/O completes, updates disk size to A+B+C> > > > > > > <power failure> > > > > > > > > > > > > After reboot, file has zero padding in range [A+B, A+B+C]: > > > > > > > > > > > > |< Block Size (BS) >| > > > > > > |DDDDDDDDDDDDDDDD0000000000000000| > > > > > > ^ ^ ^ > > > > > > A A+B A+B+C (EOF) > > > > > > > > > > > > > > > > Thanks for the diagram. FWIW, I found the description a little confusing > > > > > because A+B+C to me implies that we'd update i_size to the end of the > > > > > write from thread 2, but it seems that is only true up to the end of the > > > > > block. > > > > > > > > > > I.e., with 4k FSB and if thread 1 writes [0, 2k], then thread 2 writes > > > > > from [2, 16k], the write completion from the thread 1 write will set > > > > > i_size to 4k, not 16k, right? > > > > > > > > > > > > > Not right, the problem I'm trying to describe is: > > > > > > > > 1) thread 1 writes [0, 2k] > > > > 2) thread 2 writes [2k, 3k] > > > > 3) write completion from the thread 1 write set i_size to 3K > > > > 4) power failure > > > > 5) after reboot, [2k, 3K] of the file filled with zero and the file size is 3k > > > > > > > > > > Yeah, I get the subblock case. What I am saying above is it seems like > > > "update inode size to A+B+C" is only true for certain, select values > > > that describe the subblock case. I.e., what is the resulting i_size if > > > we replace C=1k in the example above with something >= FSB size, like > > > C=4k? > > > > > > Note this isn't all that important. I was just trying to say that the > > > overly general description made this a little more confusing to grok at > > > first than it needed to be, because to me it subtly implies there is > > > logic around somewhere that explicitly writes in-core i_size to disk, > > > when that is not actually what is happening. > > > > > > > > > > > Sorry for my previous misunderstanding. You are correct - my commit > > message description didn't cover the case where A+B+C > block size. > > In such scenarios, the final file size might end up being 4K, which > > is not what we would expect. Initially, I incorrectly thought this > > wasn't a significant issue and thus overlooked this case. Let me > > update the diagram to address this. > > > > Ok no problem.. like I said, just a minor nit. ;) > > > Thread 1: Thread 2: > > ------------ ----------- > > write [A, A+B] > > update inode size to A+B > > submit I/O [A, A+BS] > > write [A+B, A+B+C] > > update inode size to A+B+C > > <I/O completes, updates disk size to A+B+C> > > <power failure> > > > > After reboot: > > 1) The file has zero padding in the range [A+B, A+BS] > > 2) The file size is unexpectedly set to A+BS > > > > |< Block Size (BS) >|< Block Size (BS) >| > > |DDDDDDDDDDDDDDDD0000000000000000|00000000000000000000000000000000| > > ^ ^ ^ ^ > > A A+B A+BS (EOF) A+B+C > > > > > > It will be update in the next version. > > > > The text above still says "updates disk size to A+B+C." I'm not sure if > you intended to change that to A+BS as well, but regardless LGTM. > Thanks. > Yes, forgot to update here. Thanks, Long Li
