On Wed 27-09-17 07:00:53, Dan Williams wrote: > On Wed, Sep 27, 2017 at 4:35 AM, Jan Kara <jack@xxxxxxx> wrote: > > 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. > > We can also get rid of VM_MIXEDMAP if we disable DAX in the > !pfn_t_has_page() case. Yeah, although it would be a pity to require struct page just to avoid having to set VM_MIXEDMAP flag... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
