On Wed, May 29, 2024 at 09:54:30AM -0300, Fabiano Rosas wrote: > Nicholas Piggin <npig...@gmail.com> writes: > > > Postcopy requires userfaultfd support, which requires tmpfs if a memory > > file is used. > > > > This adds back support for /dev/shm memory files, but adds preallocation > > to skip environments where that mount is limited in size. > > > > Signed-off-by: Nicholas Piggin <npig...@gmail.com> > > --- > > > > How about this? This goes on top of the reset of the patches > > (I'll re-send them all as a series if we can get to some agreement). > > > > This adds back the /dev/shm option with preallocation and adds a test > > case that requires tmpfs. > > Peter has stronger opinions on this than I do. I'll leave it to him to > decide.
Sorry if I gave that feeling; it's more of a stronger willingness to at some point enable shmem for QEMU migration, rather than wanting to push back what Nicholas was trying to do. Enabling more arch for migration tests is definitely worthwhile on its own. Shmem is just some blank spot that IMHO we should start to think about better coverarge. E.g. it is the only sane way to boot the VM that is able to do fast qemu upgrades using ignore-shared, that was true even before Steve's cpr-exec work, which would be much easier than anonymous. And it's also possible shmem can be (in the next 3-5 years) the 1G page provider to replace hugetlb for postcopy's sake - this one is far beyond our current discussion so I won't extend.. IMHO shmem should just be a major backend just like anonymous, and the only possible file backend we can test in CI - as hugetlb is harder to manage there. > > Just note that now we're making the CI less deterministic in relation to > the migration tests. When a test that uses shmem fails, we'll not be > able to consistently reproduce because the test might not even run > depending on what has consumed the shmem first. > > Let's also take care that the other consumers of shmem (I think just > ivshmem-test) are able to cope with the migration-test taking all the > space, otherwise the CI will still break. Looks like ivshmem-test only uses 1MB shmem constantly so probably that will succeed if the migration test will, but true they face the same challenge and they interfere with each other.. that test sidently pass (instead of skip) if mktempshm() fails. I guess we don't have a way to solidly test shmem as shmem simply may not be around. For this patch alone personally I'd avoid using "use_uffd_memfile" as the name, as that's definitely confusing, since shmem can be tested in other setups too without uffd. Nicolas, please feel free to move ahead with your arch enablement series with /tmp if you want to separate the shmem issue. Thanks, -- Peter Xu
