Messages in this thread | | | Date | Mon, 17 Oct 2011 03:49:18 -0400 | From | Christoph Hellwig <> | Subject | Re: [PATCH] loop: remove the incorrect write_begin/write_end shortcut |
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On Tue, Sep 20, 2011 at 12:09:22PM -0400, Christoph Hellwig wrote: > Currently the loop device tries to call directly into write_begin/write_end > instead of going through ->write if it can. This is a fairly nasty shortcut > as write_begin and write_end are only callbacks for the generic write code > and expect to be called with filesystem specific locks held. > > This code currently causes various issues for clustered filesystems as it > doesn't take the required cluster locks, and it also causes issues for XFS > as it doesn't properly lock against the swapext ioctl as called by the > defragmentation tools. This in case causes data corruption if > defragmentation hits a busy loop device in the wrong time window, as > reported by RH QA. > > The reason why we have this shortcut is that it saves a data copy when > doing a transformation on the loop device, which is the technical term > for using cryptoloop (or an XOR transformation). Given that cryptoloop > has been deprecated in favour of dm-crypt my opinion is that we should > simply drop this shortcut instead of finding complicated ways to to > introduce a formal interface for this shortcut. > > Signed-off-by: Christoph Hellwig <hch@lst.de> > > Index: linux-2.6/drivers/block/loop.c > =================================================================== > --- linux-2.6.orig/drivers/block/loop.c 2011-09-19 12:21:06.725231729 -0400 > +++ linux-2.6/drivers/block/loop.c 2011-09-19 12:28:01.671730150 -0400 > @@ -203,74 +203,6 @@ lo_do_transfer(struct loop_device *lo, i > } > > /** > - * do_lo_send_aops - helper for writing data to a loop device > - * > - * This is the fast version for backing filesystems which implement the address > - * space operations write_begin and write_end. > - */ > -static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec, > - loff_t pos, struct page *unused) > -{ > - struct file *file = lo->lo_backing_file; /* kudos to NFsckingS */ > - struct address_space *mapping = file->f_mapping; > - pgoff_t index; > - unsigned offset, bv_offs; > - int len, ret; > - > - mutex_lock(&mapping->host->i_mutex); > - index = pos >> PAGE_CACHE_SHIFT; > - offset = pos & ((pgoff_t)PAGE_CACHE_SIZE - 1); > - bv_offs = bvec->bv_offset; > - len = bvec->bv_len; > - while (len > 0) { > - sector_t IV; > - unsigned size, copied; > - int transfer_result; > - struct page *page; > - void *fsdata; > - > - IV = ((sector_t)index << (PAGE_CACHE_SHIFT - 9))+(offset >> 9); > - size = PAGE_CACHE_SIZE - offset; > - if (size > len) > - size = len; > - > - ret = pagecache_write_begin(file, mapping, pos, size, 0, > - &page, &fsdata); > - if (ret) > - goto fail; > - > - file_update_time(file); > - > - transfer_result = lo_do_transfer(lo, WRITE, page, offset, > - bvec->bv_page, bv_offs, size, IV); > - copied = size; > - if (unlikely(transfer_result)) > - copied = 0; > - > - ret = pagecache_write_end(file, mapping, pos, size, copied, > - page, fsdata); > - if (ret < 0 || ret != copied) > - goto fail; > - > - if (unlikely(transfer_result)) > - goto fail; > - > - bv_offs += copied; > - len -= copied; > - offset = 0; > - index++; > - pos += copied; > - } > - ret = 0; > -out: > - mutex_unlock(&mapping->host->i_mutex); > - return ret; > -fail: > - ret = -1; > - goto out; > -} > - > -/** > * __do_lo_send_write - helper for writing data to a loop device > * > * This helper just factors out common code between do_lo_send_direct_write() > @@ -297,10 +229,8 @@ static int __do_lo_send_write(struct fil > /** > * do_lo_send_direct_write - helper for writing data to a loop device > * > - * This is the fast, non-transforming version for backing filesystems which do > - * not implement the address space operations write_begin and write_end. > - * It uses the write file operation which should be present on all writeable > - * filesystems. > + * This is the fast, non-transforming version that does not need double > + * buffering. > */ > static int do_lo_send_direct_write(struct loop_device *lo, > struct bio_vec *bvec, loff_t pos, struct page *page) > @@ -316,15 +246,9 @@ static int do_lo_send_direct_write(struc > /** > * do_lo_send_write - helper for writing data to a loop device > * > - * This is the slow, transforming version for filesystems which do not > - * implement the address space operations write_begin and write_end. It > - * uses the write file operation which should be present on all writeable > - * filesystems. > - * > - * Using fops->write is slower than using aops->{prepare,commit}_write in the > - * transforming case because we need to double buffer the data as we cannot do > - * the transformations in place as we do not have direct access to the > - * destination pages of the backing file. > + * This is the slow, transforming version that needs to double buffer the > + * data as it cannot do the transformations in place without having direct > + * access to the destination pages of the backing file. > */ > static int do_lo_send_write(struct loop_device *lo, struct bio_vec *bvec, > loff_t pos, struct page *page) > @@ -350,17 +274,16 @@ static int lo_send(struct loop_device *l > struct page *page = NULL; > int i, ret = 0; > > - do_lo_send = do_lo_send_aops; > - if (!(lo->lo_flags & LO_FLAGS_USE_AOPS)) { > + if (lo->transfer != transfer_none) { > + page = alloc_page(GFP_NOIO | __GFP_HIGHMEM); > + if (unlikely(!page)) > + goto fail; > + kmap(page); > + do_lo_send = do_lo_send_write; > + } else { > do_lo_send = do_lo_send_direct_write; > - if (lo->transfer != transfer_none) { > - page = alloc_page(GFP_NOIO | __GFP_HIGHMEM); > - if (unlikely(!page)) > - goto fail; > - kmap(page); > - do_lo_send = do_lo_send_write; > - } > } > + > bio_for_each_segment(bvec, bio, i) { > ret = do_lo_send(lo, bvec, pos, page); > if (ret < 0) > @@ -849,35 +772,23 @@ static int loop_set_fd(struct loop_devic > mapping = file->f_mapping; > inode = mapping->host; > > - if (!(file->f_mode & FMODE_WRITE)) > - lo_flags |= LO_FLAGS_READ_ONLY; > - > error = -EINVAL; > - if (S_ISREG(inode->i_mode) || S_ISBLK(inode->i_mode)) { > - const struct address_space_operations *aops = mapping->a_ops; > - > - if (aops->write_begin) > - lo_flags |= LO_FLAGS_USE_AOPS; > - if (!(lo_flags & LO_FLAGS_USE_AOPS) && !file->f_op->write) > - lo_flags |= LO_FLAGS_READ_ONLY; > + if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode)) > + goto out_putf; > > - lo_blocksize = S_ISBLK(inode->i_mode) ? > - inode->i_bdev->bd_block_size : PAGE_SIZE; > + if (!(file->f_mode & FMODE_WRITE) || !(mode & FMODE_WRITE) || > + !file->f_op->write) > + lo_flags |= LO_FLAGS_READ_ONLY; > > - error = 0; > - } else { > - goto out_putf; > - } > + lo_blocksize = S_ISBLK(inode->i_mode) ? > + inode->i_bdev->bd_block_size : PAGE_SIZE; > > + error = -EFBIG; > size = get_loop_size(lo, file); > - > - if ((loff_t)(sector_t)size != size) { > - error = -EFBIG; > + if ((loff_t)(sector_t)size != size) > goto out_putf; > - } > > - if (!(mode & FMODE_WRITE)) > - lo_flags |= LO_FLAGS_READ_ONLY; > + error = 0; > > set_device_ro(bdev, (lo_flags & LO_FLAGS_READ_ONLY) != 0); > > Index: linux-2.6/include/linux/loop.h > =================================================================== > --- linux-2.6.orig/include/linux/loop.h 2011-09-19 12:21:32.241231706 -0400 > +++ linux-2.6/include/linux/loop.h 2011-09-19 12:21:40.768232536 -0400 > @@ -73,7 +73,6 @@ struct loop_device { > */ > enum { > LO_FLAGS_READ_ONLY = 1, > - LO_FLAGS_USE_AOPS = 2, > LO_FLAGS_AUTOCLEAR = 4, > }; > ---end quoted text---
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