On 09/02/2015 12:13 PM, Kirill A. Shutemov wrote: > On Wed, Sep 02, 2015 at 08:49:22AM +1000, Dave Chinner wrote: >> On Tue, Sep 01, 2015 at 01:08:04PM +0300, Kirill A. Shutemov wrote: >>> On Tue, Sep 01, 2015 at 09:38:03AM +1000, Dave Chinner wrote: >>>> On Mon, Aug 31, 2015 at 12:59:44PM -0600, Ross Zwisler wrote: >>>> Even for DAX, msync has to call vfs_fsync_range() for the filesystem to commit >>>> the backing store allocations to stable storage, so there's not >>>> getting around the fact msync is the wrong place to be flushing >>>> DAX mappings to persistent storage. >>> >>> Why? >>> IIUC, msync() doesn't have any requirements wrt metadata, right? >> >> Of course it does. If the backing store allocation has not been >> committed, then after a crash there will be a hole in file and >> so it will read as zeroes regardless of what data was written and >> flushed. > > Any reason why backing store allocation cannot be committed on *_mkwrite? > >>>> I pointed this out almost 6 months ago (i.e. that fsync was broken) >>>> anf hinted at how to solve it. Fix fsync, and msync gets fixed for >>>> free: >>>> >>>> https://lists.01.org/pipermail/linux-nvdimm/2015-March/000341.html >>>> >>>> I've also reported to Willy that DAX write page faults don't work >>>> correctly, either. xfstests generic/080 exposes this: a read >>>> from a page followed immediately by a write to that page does not >>>> result in ->page_mkwrite being called on the write and so >>>> backing store is not allocated for the page, nor are the timestamps >>>> for the file updated. This will also result in fsync (and msync) >>>> not working properly. >>> >>> Is that because XFS doesn't provide vm_ops->pfn_mkwrite? >> >> I didn't know that had been committed. I don't recall seeing a pull >> request with that in it > > It went though -mm tree. > >> none of the XFS DAX patches conflicted >> against it and there's been no runtime errors. I'll fix it up. >> >> As such, shouldn't there be a check in the VM (in ->mmap callers) >> that if we have the vma is returned with VM_MIXEDMODE enabled that >> ->pfn_mkwrite is not NULL? It's now clear to me that any filesystem >> that sets VM_MIXEDMODE needs to support both page_mkwrite and >> pfn_mkwrite, and such a check would have caught this immediately... > > I guess it's "both or none" case. We have VM_MIXEDMAP users who don't care > about *_mkwrite. > > I'm not yet sure it would be always correct, but something like this will > catch the XFS case, without false-positive on other stuff in my KVM setup: > > diff --git a/mm/mmap.c b/mm/mmap.c > index 3f78bceefe5a..f2e29a541e14 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -1645,6 +1645,15 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > vma->vm_ops = &dummy_ops; > } > > + /* > + * Make sure that for VM_MIXEDMAP VMA has both > + * vm_ops->page_mkwrite and vm_ops->pfn_mkwrite or has none. > + */ > + if ((vma->vm_ops->page_mkwrite || vma->vm_ops->pfn_mkwrite) && > + vma->vm_flags & VM_MIXEDMAP) { > + VM_BUG_ON_VMA(!vma->vm_ops->page_mkwrite, vma); > + VM_BUG_ON_VMA(!vma->vm_ops->pfn_mkwrite, vma); BTW: the page_mkwrite is used for reading of holes that put zero-pages at the radix tree. One can just map a single global zero-page in pfn-mode for that. Kirill Hi. Please don't make these BUG_ONs its counter productive believe me. Please make them WARN_ON_ONCE() it is not a crashing bug to work like this. (Actually it is not a bug at all in some cases, but we can relax that when a user comes up) Thanks Boaz > + } > addr = vma->vm_start; > vm_flags = vma->vm_flags; > } else if (vm_flags & VM_SHARED) { > -- 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>