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E.g. iomap skips over pages > reported as holes, and ext4_writepage complains about pages without > buffers and then clears the dirty bit and continues. > > I'm kinda surprised that btrfs wants to treat this so special > especially as more of the btrfs page and sub-page status will be out > of date as well. As Sterba points out later in the thread, btrfs cares more because of stable page requirements to protect data during COW and to make sure the crcs we write to disk are correct. The fixup worker path is pretty easy to trigger if you O_DIRECT reads into mmap'd pages. You need some memory pressure to power through get_user_pages trying to do the right thing, but it does happen. I'd love a proper fix for this on the *_user_pages() side where page_mkwrite() style notifications are used all the time. It's just a huge change, and my answer so far has always been that using btrfs mmap'd memory for this kind of thing isn't a great choice either way. -chris