On 7/15/25 9:34 PM, hanqi wrote: > > > ? 2025/7/15 22:28, Jens Axboe ??: >> On 7/14/25 9:10 PM, Qi Han wrote: >>> Jens has already completed the development of uncached buffered I/O >>> in git [1], and in f2fs, the feature can be enabled simply by setting >>> the FOP_DONTCACHE flag in f2fs_file_operations. >> You need to ensure that for any DONTCACHE IO that the completion is >> routed via non-irq context, if applicable. I didn't verify that this is >> the case for f2fs. Generally you can deduce this as well through >> testing, I'd say the following cases would be interesting to test: >> >> 1) Normal DONTCACHE buffered read >> 2) Overwrite DONTCACHE buffered write >> 3) Append DONTCACHE buffered write >> >> Test those with DEBUG_ATOMIC_SLEEP set in your config, and it that >> doesn't complain, that's a great start. >> >> For the above test cases as well, verify that page cache doesn't grow as >> IO is performed. A bit is fine for things like meta data, but generally >> you want to see it remain basically flat in terms of page cache usage. >> >> Maybe this is all fine, like I said I didn't verify. Just mentioning it >> for completeness sake. > > Hi, Jens > Thanks for your suggestion. As I mentioned earlier in [1], in f2fs, > the regular buffered write path invokes folio_end_writeback from a > softirq context. Therefore, it seems that f2fs may not be suitable > for DONTCACHE I/O writes. > > I?d like to ask a question: why is DONTCACHE I/O write restricted to > non-interrupt context only? Is it because dropping the page might be > too time-consuming to be done safely in interrupt context? This might > be a naive question, but I?d really appreciate your clarification. > Thanks in advance.
Because (as of right now, at least) the code doing the invalidation needs process context. There are various reasons for this, which you'll see if you follow the path off folio_end_writeback() -> filemap_end_dropbehind_write() -> filemap_end_dropbehind() -> folio_unmap_invalidate(). unmap_mapping_folio() is one case, and while that may be doable, the inode i_lock is not IRQ safe. Most file systems have a need to punt some writeback completions to non-irq context, eg for file extending etc. Hence for most file systems, the dontcache case just becomes another case that needs to go through that path. It'd certainly be possible to improve upon this, for example by having an opportunistic dontcache unmap from IRQ/soft-irq context, and then punting to a workqueue if that doesn't pan out. But this doesn't exist as of yet, hence the need for the workqueue punt. -- Jens Axboe _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
