On Fri, 24 Oct 2014 17:20:35 -0400 Matthew Wilcox <[email protected]> wrote:
> Pagecache faults recheck i_size after taking the page lock to ensure that > the fault didn't race against a truncate. We don't have a page to lock > in the XIP case, so use the i_mmap_mutex instead. It is locked in the > truncate path in unmap_mapping_range() after updating i_size. So while > we hold it in the fault path, we are guaranteed that either i_size has > already been updated in the truncate path, or that the truncate will > subsequently call zap_page_range_single() and so remove the mapping we > have just inserted. > > There is a window of time in which i_size has been reduced and the > thread has a mapping to a page which will be removed from the file, > but this is harmless as the page will not be allocated to a different > purpose before the thread's access to it is revoked. > i_mmap_mutex is no more. I made what are hopefulyl the appropriate changes. Also, that new locking rule is pretty subtle and we need to find a way of alerting readers (and modifiers) of mm/memory.c to DAX's use of i_mmap_lock(). Please review my suggested addition for accuracy and cmopleteness. From: Andrew Morton <[email protected]> Subject: mm-fix-xip-fault-vs-truncate-race-fix switch to i_mmap_lock_read(), add comment in unmap_single_vma() Cc: Jan Kara <[email protected]> Cc: Kirill A. Shutemov <[email protected]> Cc: Mathieu Desnoyers <[email protected]> Cc: Matthew Wilcox <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- mm/filemap_xip.c | 20 +++++++++++++------- mm/memory.c | 5 +++++ 2 files changed, 18 insertions(+), 7 deletions(-) diff -puN mm/filemap_xip.c~mm-fix-xip-fault-vs-truncate-race-fix mm/filemap_xip.c --- a/mm/filemap_xip.c~mm-fix-xip-fault-vs-truncate-race-fix +++ a/mm/filemap_xip.c @@ -255,17 +255,20 @@ again: __xip_unmap(mapping, vmf->pgoff); found: - /* We must recheck i_size under i_mmap_mutex */ - mutex_lock(&mapping->i_mmap_mutex); + /* + * We must recheck i_size under i_mmap_rwsem to prevent races + * with truncation + */ + i_mmap_lock_read(mapping); size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT; if (unlikely(vmf->pgoff >= size)) { - mutex_unlock(&mapping->i_mmap_mutex); + i_mmap_unlock_read(mapping); return VM_FAULT_SIGBUS; } err = vm_insert_mixed(vma, (unsigned long)vmf->virtual_address, xip_pfn); - mutex_unlock(&mapping->i_mmap_mutex); + i_mmap_unlock_read(mapping); if (err == -ENOMEM) return VM_FAULT_OOM; /* @@ -290,8 +293,11 @@ found: if (error != -ENODATA) goto out; - /* We must recheck i_size under i_mmap_mutex */ - mutex_lock(&mapping->i_mmap_mutex); + /* + * We must recheck i_size under i_mmap_rwsem to prevent races + * with truncation + */ + i_mmap_lock_read(mapping); size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT; if (unlikely(vmf->pgoff >= size)) { @@ -309,7 +315,7 @@ found: ret = VM_FAULT_NOPAGE; unlock: - mutex_unlock(&mapping->i_mmap_mutex); + i_mmap_unlock_read(mapping); out: write_seqcount_end(&xip_sparse_seq); mutex_unlock(&xip_sparse_mutex); diff -puN mm/memory.c~mm-fix-xip-fault-vs-truncate-race-fix mm/memory.c --- a/mm/memory.c~mm-fix-xip-fault-vs-truncate-race-fix +++ a/mm/memory.c @@ -1327,6 +1327,11 @@ static void unmap_single_vma(struct mmu_ * safe to do nothing in this case. */ if (vma->vm_file) { + /* + * Note that DAX uses i_mmap_lock to serialise + * against file truncate - truncate calls into + * unmap_single_vma(). + */ i_mmap_lock_write(vma->vm_file->f_mapping); __unmap_hugepage_range_final(tlb, vma, start, end, NULL); i_mmap_unlock_write(vma->vm_file->f_mapping); _ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
