RE: [PATCH v3 1/4] fs: allow cross-FS copy_file_range for memory-backed files

[wangtao <tao.wangtao@honor.com>]

> -----Original Message-----
> From: Amir Goldstein <amir73il@gmail.com>
> Sent: Friday, May 30, 2025 9:44 PM
> To: wangtao <tao.wangtao@honor.com>
> Cc: sumit.semwal@linaro.org; christian.koenig@amd.com;
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> mm@kvack.org; wangbintian(BintianWang) <bintian.wang@honor.com>;
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> Subject: Re: [PATCH v3 1/4] fs: allow cross-FS copy_file_range for memory-
> backed files
> 
> On Fri, May 30, 2025 at 12:40 PM wangtao <tao.wangtao@honor.com> wrote:
> >
> > Memory-backed files can optimize copy performance via copy_file_range
> > callbacks. Compared to mmap&read: reduces GUP (get_user_pages)
> > overhead; vs sendfile/splice: eliminates one memory copy; supports
> > dmabuf zero-copy implementation.
> >
> > Signed-off-by: wangtao <tao.wangtao@honor.com>
> > ---
> 
> Hi wangtao,
> 
> Let me rephrase my reaction gently:
> Regardless of the proposed API extension, and referring to the
> implementation itself - I wrote the current code and I can barely understand
> the logic every time I come back to read it.
> 
> Your changes make it too messy to be reviewed/maintained.
> I have a few suggestions for simplifications (see below).
> 
> The complication arise from the fact that normally the destination fops are
> used to call the copy_range() method and this case is a deviation from this
> standard, we need to make the cope simpler using a helper to choose the
> fops to use.
> 
> >  fs/read_write.c    | 71 +++++++++++++++++++++++++++++++++-----------
> --
> >  include/linux/fs.h |  2 ++
> >  2 files changed, 54 insertions(+), 19 deletions(-)
> >
> > diff --git a/fs/read_write.c b/fs/read_write.c index
> > bb0ed26a0b3a..591c6db7b785 100644
> > --- a/fs/read_write.c
> > +++ b/fs/read_write.c
> > @@ -1469,6 +1469,20 @@ COMPAT_SYSCALL_DEFINE4(sendfile64, int,
> out_fd,
> > int, in_fd,  }  #endif
> >
> > +static inline bool is_copy_memory_file_to_file(struct file *file_in,
> > +                               struct file *file_out) {
> > +       return (file_in->f_op->fop_flags & FOP_MEMORY_FILE) &&
> > +               file_in->f_op->copy_file_range &&
> > +file_out->f_op->write_iter; }
> > +
> > +static inline bool is_copy_file_to_memory_file(struct file *file_in,
> > +                               struct file *file_out) {
> > +       return (file_out->f_op->fop_flags & FOP_MEMORY_FILE) &&
> > +               file_in->f_op->read_iter &&
> > +file_out->f_op->copy_file_range; }
> > +
> 
> Please combine that to a helper:
> const struct file_operations *memory_copy_file_ops(struct file *file_in,
> struct file *file_out) which returns either file_in->f_op, file_out->f_op or
> NULL.
> 
Great suggestions! I'll update them in the new patch version soon.

> 
> >  /*
> >   * Performs necessary checks before doing a file copy
> >   *
> > @@ -1484,11 +1498,23 @@ static int generic_copy_file_checks(struct file
> *file_in, loff_t pos_in,
> >         struct inode *inode_out = file_inode(file_out);
> >         uint64_t count = *req_count;
> >         loff_t size_in;
> > +       bool splice = flags & COPY_FILE_SPLICE;
> > +       bool has_memory_file;
> >         int ret;
> >
> > -       ret = generic_file_rw_checks(file_in, file_out);
> > -       if (ret)
> > -               return ret;
> > +       /* Skip generic checks, allow cross-sb copies for dma-buf/tmpfs */
> > +       has_memory_file = is_copy_memory_file_to_file(file_in, file_out) ||
> > +                         is_copy_file_to_memory_file(file_in, file_out);
> > +       if (!splice && has_memory_file) {
> > +               if (!(file_in->f_mode & FMODE_READ) ||
> > +                   !(file_out->f_mode & FMODE_WRITE) ||
> > +                   (file_out->f_flags & O_APPEND))
> > +                       return -EBADF;
> 
> I don't like that this both duplicates code and does not check all the checks in
> generic_file_rw_checks() for the non-memory file side.
> I do not currently have a suggestion how to make this less messy, more
> human readable and correct.
Since dmabuf files aren't regular files, we'll skip generic_file_rw_checks.
Adding an essential_file_rw_checks() could reduce code duplication.

> 
> > +       } else {
> > +               ret = generic_file_rw_checks(file_in, file_out);
> > +               if (ret)
> > +                       return ret;
> > +       }
> >
> >         /*
> >          * We allow some filesystems to handle cross sb copy, but
> > passing @@ -1500,7 +1526,7 @@ static int generic_copy_file_checks(struct
> file *file_in, loff_t pos_in,
> >          * and several different sets of file_operations, but they all end up
> >          * using the same ->copy_file_range() function pointer.
> >          */
> > -       if (flags & COPY_FILE_SPLICE) {
> > +       if (splice || has_memory_file) {
> >                 /* cross sb splice is allowed */
> 
> This comment is not accurate - it should say "cross fs-type", because it skips
> the test that compares the ->copy_file_range pointers, not the sb.
> If you are making changes to this function, this should be clarified.
> 
Will fix this shortly.

> >         } else if (file_out->f_op->copy_file_range) {
> >                 if (file_in->f_op->copy_file_range != @@ -1581,23
> > +1607,30 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
> >          * same sb using clone, but for filesystems where both clone and copy
> >          * are supported (e.g. nfs,cifs), we only call the copy method.
> >          */
> > -       if (!splice && file_out->f_op->copy_file_range) {
> > -               ret = file_out->f_op->copy_file_range(file_in, pos_in,
> > -                                                     file_out, pos_out,
> > -                                                     len, flags);
> > -       } else if (!splice && file_in->f_op->remap_file_range && samesb) {
> > -               ret = file_in->f_op->remap_file_range(file_in, pos_in,
> > -                               file_out, pos_out,
> > -                               min_t(loff_t, MAX_RW_COUNT, len),
> > -                               REMAP_FILE_CAN_SHORTEN);
> > -               /* fallback to splice */
> > -               if (ret <= 0)
> > +       if (!splice) {
> > +               if (is_copy_memory_file_to_file(file_in, file_out)) {
> > +                       ret = file_in->f_op->copy_file_range(file_in, pos_in,
> > +                                       file_out, pos_out, len, flags);
> > +               } else if (is_copy_file_to_memory_file(file_in, file_out)) {
> > +                       ret = file_out->f_op->copy_file_range(file_in, pos_in,
> > +                                       file_out, pos_out, len, flags);
> > +               } else if (file_out->f_op->copy_file_range) {
> > +                       ret = file_out->f_op->copy_file_range(file_in, pos_in,
> > +                                                       file_out, pos_out,
> > +                                                       len, flags);
> > +               } else if (file_in->f_op->remap_file_range && samesb) {
> > +                       ret = file_in->f_op->remap_file_range(file_in, pos_in,
> > +                                       file_out, pos_out,
> > +                                       min_t(loff_t, MAX_RW_COUNT, len),
> > +                                       REMAP_FILE_CAN_SHORTEN);
> > +                       /* fallback to splice */
> > +                       if (ret <= 0)
> > +                               splice = true;
> > +               } else if (samesb) {
> > +                       /* Fallback to splice for same sb copy for
> > + backward compat */
> >                         splice = true;
> > -       } else if (samesb) {
> > -               /* Fallback to splice for same sb copy for backward compat */
> > -               splice = true;
> > +               }
> 
> This is the way-too-ugly-to-live part.
> Was pretty bad before and now it is unacceptable.
> way too much unneeded nesting and too hard to follow.
> 
> First of all, please use splice label and call goto splice (before the comment)
> instead of adding another nesting level.
> I would do that even if not adding memory fd copy support.
> 
> Second, please store result of mem_fops = memory_copy_file_ops() and
> start with:
> +    if (mem_fops) {
> +        ret = mem_fops->copy_file_range(file_in, pos_in,
> +
> file_out, pos_out,
> +                                                               len,
> + flags);
>  +   } else if (file_out->f_op->copy_file_range) { ...
> 
> and update the comment above to say that:
> + * For copy to/from memory fd, we alway call the copy method of the
> memory fd.
>    */
> 
> I think that makes the code not more ugly than it already is.
> 
> Thanks,
> Amir.
Good advice - thanks a lot!

Regards,
Wangtao.