Messages in this thread |  | | Date | Tue, 26 Oct 2021 01:12:53 -0400 | From | "Theodore Ts'o" <> | Subject | Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks |
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On Mon, Oct 25, 2021 at 08:24:26PM +0200, Andreas Gruenbacher wrote: > > For generic_perform_write() Dave Hansen attempted to move the fault-in > > after the uaccess in commit 998ef75ddb57 ("fs: do not prefault > > sys_write() user buffer pages"). This was reverted as it was exposing an > > ext4 bug. I don't [know] whether it was fixed but re-applying Dave's commit > > avoids the performance drop. > > Interesting. The revert of commit 998ef75ddb57 is in commit > 00a3d660cbac. Maybe Dave and Ted can tell us more about what went > wrong in ext4 and whether it's still an issue.
The context for the revert can be found here[1].
[1] https://lore.kernel.org/lkml/20151005152236.GA8140@thunk.org/
And "what went wrong in ext4" was fixed here[2].
[2] https://lore.kernel.org/lkml/20151005152236.GA8140@thunk.org/
which landed upstream as commit b90197b65518 ("ext4: use private version of page_zero_new_buffers() for data=journal mode").
So it looks like the original issue which triggered the revert in 2015 should be addressed, and we can easily test it by using generic/208 with data=journal mode.
There also seems to be a related discussion about whether we should unrevert 998ef75ddb57 here[3]. Hmm. there is a mention on that thread in [3], "Side note: search for "iov_iter_fault_in_writeable()" on lkml for a gfs2 patch-series that is buggy, exactly because it does *not* use the atomic user space accesses, and just tries to do the fault-in to hide the real bug." I assume that's related to the discussion on this thread?
[3] https://lore.kernel.org/all/3221175.1624375240@warthog.procyon.org.uk/T/#u
- Ted
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