On 05/15/2015 01:09 AM, Mel Gorman wrote: > On Thu, May 14, 2015 at 11:06:22PM -0400, Rik van Riel wrote: >> On 05/14/2015 08:06 PM, Daniel Phillips wrote: >>>> The issue is that things like ptrace, AIO, infiniband >>>> RDMA, and other direct memory access subsystems can take >>>> a reference to page A, which Tux3 clones into a new page B >>>> when the process writes it. >>>> >>>> However, while the process now points at page B, ptrace, >>>> AIO, infiniband, etc will still be pointing at page A. >>>> >>>> This causes the process and the other subsystem to each >>>> look at a different page, instead of at shared state, >>>> causing ptrace to do nothing, AIO and RDMA data to be >>>> invisible (or corrupted), etc... >>> >>> Is this a bit like page migration? >> >> Yes. Page migration will fail if there is an "extra" >> reference to the page that is not accounted for by >> the migration code. > > When I said it's not like page migration, I was referring to the fact > that a COW on a pinned page for RDMA is a different problem to page > migration. The COW of a pinned page can lead to lost writes or > corruption depending on the ordering of events. I see the lost writes case, but not the corruption case, Do you mean corruption by changing a page already in writeout? If so, don't all filesystems have that problem? If RDMA to a mmapped file races with write(2) to the same file, maybe it is reasonable and expected to lose some data. > Page migration fails > when there are unexpected problems to avoid this class of issue which is > fine for page migration but may be a critical failure in a filesystem > depending on exactly why the copy is required. Regards, Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
