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. -- 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
