On Tue 26-09-17 14:41:53, Dan Williams wrote: > On Tue, Sep 26, 2017 at 2:06 PM, Ross Zwisler > <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > > On Tue, Sep 26, 2017 at 12:19:21PM -0700, Dan Williams wrote: > >> On Tue, Sep 26, 2017 at 11:57 AM, Ross Zwisler > > <> > >> > This decision can only be made (in this > >> > proposed scheme) *after* the inode->i_mapping->i_mmap tree has been > >> > populated, which means we need another call into the filesystem after this > >> > insertion has happened. > >> > >> I get that, but it seems over-engineered and something that can also > >> be safely cleaned up after the fact by the code path that is disabling > >> DAX. > > > > I don't think you can safely clean it up after the fact because some thread > > might have already called ->mmap() to set up the vma->vm_flags for their new > > mapping, but they haven't added it to inode->i_mapping->i_mmap. > > If madvise(MADV_NOHUGEPAGE) can dynamically change vm_flags, then the > DAX disable path can as well. VM_MIXEDMAP looks to be a nop for normal > memory mappings. > > > The inode->i_mapping->i_mmap tree is the only way (that I know of at least) > > that the filesystem has any idea about about the mapping. This is the method > > by which we would try and clean up mapping flags, if we were to do so, and > > it's the only way that the filesystem can know whether or not mappings exist. > > > > The only way that I could think of to make this safely work is to have the > > insertion into the inode->i_mapping->i_mmap tree be our sync point. After > > that the filesystem and the mapping code can communicate on the state of DAX, > > but before that I think it's basically indeterminate. > > If we lose the race and leak VM_HUGEPAGE to a non-DAX mapping what > breaks? I'd rather be in favor of not setting VM_HUGEPAGE at all in > the ->mmap() handler and let the default THP policy take over. In > fact, see transparent_hugepage_enabled() we already auto-enable huge > page support for dax mappings regardless of VM_HUGEPAGE. Hum, this is an interesting option. So do you suggest that filesystems supporting DAX would always setup mappings with VM_MIXEDMAP and without VM_HUGEPAGE and thus we'd get rid of dependency on S_DAX flag in ->mmap? That could actually work. The only possible issue I can see is that VM_MIXEDMAP is still slightly different from normal page mappings and it could have some performance implications - e.g. copy_page_range() does more work on VM_MIXEDMAP mappings but not on normal page mappings. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
