Amir Goldstein <amir73il@xxxxxxxxx> writes: > On Mon, Feb 15, 2021 at 8:57 PM Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote: >> >> On Mon, 2021-02-15 at 19:24 +0200, Amir Goldstein wrote: >> > On Mon, Feb 15, 2021 at 6:53 PM Trond Myklebust < >> > trondmy@xxxxxxxxxxxxxxx> wrote: >> > > >> > > On Mon, 2021-02-15 at 18:34 +0200, Amir Goldstein wrote: >> > > > On Mon, Feb 15, 2021 at 5:42 PM Luis Henriques < >> > > > lhenriques@xxxxxxx> >> > > > wrote: >> > > > > >> > > > > Nicolas Boichat reported an issue when trying to use the >> > > > > copy_file_range >> > > > > syscall on a tracefs file. It failed silently because the file >> > > > > content is >> > > > > generated on-the-fly (reporting a size of zero) and >> > > > > copy_file_range >> > > > > needs >> > > > > to know in advance how much data is present. >> > > > > >> > > > > This commit restores the cross-fs restrictions that existed >> > > > > prior >> > > > > to >> > > > > 5dae222a5ff0 ("vfs: allow copy_file_range to copy across >> > > > > devices") >> > > > > and >> > > > > removes generic_copy_file_range() calls from ceph, cifs, fuse, >> > > > > and >> > > > > nfs. >> > > > > >> > > > > Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across >> > > > > devices") >> > > > > Link: >> > > > > https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@xxxxxxxxxxxx/ >> > > > > Cc: Nicolas Boichat <drinkcat@xxxxxxxxxxxx> >> > > > > Signed-off-by: Luis Henriques <lhenriques@xxxxxxx> >> > > > >> > > > Code looks ok. >> > > > You may add: >> > > > >> > > > Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx> >> > > > >> > > > I agree with Trond that the first paragraph of the commit message >> > > > could >> > > > be improved. >> > > > The purpose of this change is to fix the change of behavior that >> > > > caused the regression. >> > > > >> > > > Before v5.3, behavior was -EXDEV and userspace could fallback to >> > > > read. >> > > > After v5.3, behavior is zero size copy. >> > > > >> > > > It does not matter so much what makes sense for CFR to do in this >> > > > case (generic cross-fs copy). What matters is that nobody asked >> > > > for >> > > > this change and that it caused problems. >> > > > >> > > >> > > No. I'm saying that this patch should be NACKed unless there is a >> > > real >> > > explanation for why we give crap about this tracefs corner case and >> > > why >> > > it can't be fixed. >> > > >> > > There are plenty of reasons why copy offload across filesystems >> > > makes >> > > sense, and particularly when you're doing NAS. Clone just doesn't >> > > cut >> > > it when it comes to disaster recovery (whereas backup to a >> > > different >> > > storage unit does). If the client has to do the copy, then you're >> > > effectively doubling the load on the server, and you're adding >> > > potentially unnecessary network traffic (or at the very least you >> > > are >> > > doubling that traffic). >> > > >> > >> > I don't understand the use case you are describing. >> > >> > Which filesystem types are you talking about for source and target >> > of copy_file_range()? >> > >> > To be clear, the original change was done to support NFS/CIFS server- >> > side >> > copy and those should not be affected by this change. >> > >> >> That is incorrect: >> >> ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file >> *dst, >> u64 dst_pos, u64 count) >> { >> >> /* >> * Limit copy to 4MB to prevent indefinitely blocking an nfsd >> * thread and client rpc slot. The choice of 4MB is somewhat >> * arbitrary. We might instead base this on r/wsize, or make it >> * tunable, or use a time instead of a byte limit, or implement >> * asynchronous copy. In theory a client could also recognize a >> * limit like this and pipeline multiple COPY requests. >> */ >> count = min_t(u64, count, 1 << 22); >> return vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0); >> } >> >> You are now explicitly changing the behaviour of knfsd when the source >> and destination filesystem differ. >> >> For one thing, you are disallowing the NFSv4.2 copy offload use case of >> copying from a local filesystem to a remote NFS server. However you are >> also disallowing the copy from, say, an XFS formatted partition to an >> ext4 partition. >> > > Got it. Ugh. And I guess overlayfs may have a similar problem. > This is easy to solve with a flag COPY_FILE_SPLICE (or something) that > is internal to kernel users. > > FWIW, you may want to look at the loop in ovl_copy_up_data() > for improvements to nfsd_copy_file_range(). > > We can move the check out to copy_file_range syscall: > > if (flags != 0) > return -EINVAL; > > Leave the fallback from all filesystems and check for the > COPY_FILE_SPLICE flag inside generic_copy_file_range(). Ok, the diff bellow is just to make sure I understood your suggestion. The patch will also need to: - change nfs and overlayfs calls to vfs_copy_file_range() so that they use the new flag. - check flags in generic_copy_file_checks() to make sure only valid flags are used (COPY_FILE_SPLICE at the moment). Also, where should this flag be defined? include/uapi/linux/fs.h? Cheers, -- Luis diff --git a/fs/read_write.c b/fs/read_write.c index 75f764b43418..341d315d2a96 100644 --- a/fs/read_write.c +++ b/fs/read_write.c @@ -1383,6 +1383,13 @@ ssize_t generic_copy_file_range(struct file *file_in, loff_t pos_in, struct file *file_out, loff_t pos_out, size_t len, unsigned int flags) { + if (!(flags & COPY_FILE_SPLICE)) { + if (!file_out->f_op->copy_file_range) + return -EOPNOTSUPP; + else if (file_out->f_op->copy_file_range != + file_in->f_op->copy_file_range) + return -EXDEV; + } return do_splice_direct(file_in, &pos_in, file_out, &pos_out, len > MAX_RW_COUNT ? MAX_RW_COUNT : len, 0); } @@ -1474,9 +1481,6 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in, { ssize_t ret; - if (flags != 0) - return -EINVAL; - ret = generic_copy_file_checks(file_in, pos_in, file_out, pos_out, &len, flags); if (unlikely(ret))