> > The fact that after a block layer initiated freeze - again mostly a > > device mapper problem - one may or may not be able to successfully read > > from the filesystem is annoying. Of course one can't write, that will > > hang one immediately. But if one still has some data in the page cache > > one can still dump the contents of that file. That's at least odd > > behavior from a users POV even if for us it's cleary why that's the > > case. > > A frozen filesystem doesn't prevent read operations from occurring. Yes, that's what I was saying. I'm not disputing that. > > > And a freeze does do a sync_filesystem() and a sync_blockdev() to flush > > out any dirty data for that specific filesystem. > > Yes, it's required to do that - the whole point of freezing a > filesystem is to bring the filesystem into a *consistent physical > state on persistent storage* and to hold it in that state until it > is thawed. > > > So it would be fitting > > to give users an api that allows them to also drop the page cache > > contents. > > Not as part of a freeze operation. Yes, that's why I'd like to have a separate e.g., flag for fadvise. > > For some use-cases like the Gnome use-case one wants to do a freeze and > > drop everything that one can from the page cache for that specific > > filesystem. > > So they have to do an extra system call between FS_IOC_FREEZE and > FS_IOC_THAW. What's the problem with that? What are you trying to > optimise by colliding cache purging with FS_IOC_FREEZE? > > If the user/application/infrastructure already has to iterate all > the mounted filesystems to freeze them, then it's trivial for them > to add a cache purging step to that infrastructure for the storage > configurations that might need it. I just don't see why this needs > to be part of a block device freeze operation, especially as the > "purge caches on this filesystem" operation has potential use cases > outside of the luksSuspend context.... Ah, I'm sorry I think we're accidently talking past each other... I'm _not_ trying to tie block layer freezing and cache purging. I'm trying to expose something like: posix_fadvise(fs_fd, [...], POSIX_FADV_FS_DONTNEED/DROP); The Gnome people could then do: cryptsetup luksSuspend posix_fadvise(fs_fd, [...], POSIX_FADV_FS_DONTNEED/DROP); as two separate operations. Because the dropping the caches step is useful to other users as well; completely independent of the block layer freeze that I used to motivate this.
