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    SubjectRe: [Patch v2 13/15] CIFS: Add support for direct I/O read
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    On 5/30/2018 3:48 PM, Long Li wrote:
    > From: Long Li <longli@microsoft.com>
    >
    > Implement the function for direct I/O read. It doesn't support AIO, which
    > will be implemented in a follow up patch.
    >
    > Signed-off-by: Long Li <longli@microsoft.com>
    > ---
    > fs/cifs/cifsfs.h | 1 +
    > fs/cifs/file.c | 149 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
    > 2 files changed, 150 insertions(+)
    >
    > diff --git a/fs/cifs/cifsfs.h b/fs/cifs/cifsfs.h
    > index 5f02318..7fba9aa 100644
    > --- a/fs/cifs/cifsfs.h
    > +++ b/fs/cifs/cifsfs.h
    > @@ -102,6 +102,7 @@ extern int cifs_open(struct inode *inode, struct file *file);
    > extern int cifs_close(struct inode *inode, struct file *file);
    > extern int cifs_closedir(struct inode *inode, struct file *file);
    > extern ssize_t cifs_user_readv(struct kiocb *iocb, struct iov_iter *to);
    > +extern ssize_t cifs_direct_readv(struct kiocb *iocb, struct iov_iter *to);
    > extern ssize_t cifs_strict_readv(struct kiocb *iocb, struct iov_iter *to);
    > extern ssize_t cifs_user_writev(struct kiocb *iocb, struct iov_iter *from);
    > extern ssize_t cifs_strict_writev(struct kiocb *iocb, struct iov_iter *from);
    > diff --git a/fs/cifs/file.c b/fs/cifs/file.c
    > index 87eece6..e6e6f24 100644
    > --- a/fs/cifs/file.c
    > +++ b/fs/cifs/file.c
    > @@ -2955,6 +2955,18 @@ cifs_read_allocate_pages(struct cifs_readdata *rdata, unsigned int nr_pages)
    > return rc;
    > }
    >
    > +static void cifs_direct_readdata_release(struct kref *refcount)
    > +{
    > + struct cifs_readdata *rdata = container_of(refcount,
    > + struct cifs_readdata, refcount);
    > + unsigned int i;
    > +
    > + for (i = 0; i < rdata->nr_pages; i++)
    > + put_page(rdata->pages[i]);
    > +
    > + cifs_readdata_release(refcount);
    > +}
    > +
    > static void
    > cifs_uncached_readdata_release(struct kref *refcount)
    > {
    > @@ -3267,6 +3279,143 @@ collect_uncached_read_data(struct cifs_aio_ctx *ctx)
    > complete(&ctx->done);
    > }
    >
    > +static void cifs_direct_readv_complete(struct work_struct *work)
    > +{
    > + struct cifs_readdata *rdata =
    > + container_of(work, struct cifs_readdata, work);
    > +
    > + complete(&rdata->done);
    > + kref_put(&rdata->refcount, cifs_direct_readdata_release);
    > +}
    > +
    > +ssize_t cifs_direct_readv(struct kiocb *iocb, struct iov_iter *to)
    > +{
    > + size_t len, cur_len, start;
    > + unsigned int npages, rsize, credits;
    > + struct file *file;
    > + struct cifs_sb_info *cifs_sb;
    > + struct cifsFileInfo *cfile;
    > + struct cifs_tcon *tcon;
    > + struct page **pagevec;
    > + ssize_t rc, total_read = 0;
    > + struct TCP_Server_Info *server;
    > + loff_t offset = iocb->ki_pos;
    > + pid_t pid;
    > + struct cifs_readdata *rdata;
    > +
    > + /*
    > + * iov_iter_get_pages_alloc() doesn't work with ITER_KVEC,
    > + * fall back to data copy read path
    > + */
    > + if (to->type & ITER_KVEC) {
    > + cifs_dbg(FYI, "use non-direct cifs_user_readv for kvec I/O\n");
    > + return cifs_user_readv(iocb, to);
    > + }
    > +
    > + len = iov_iter_count(to);
    > + if (!len)
    > + return 0;
    > +
    > + file = iocb->ki_filp;
    > + cifs_sb = CIFS_FILE_SB(file);
    > + cfile = file->private_data;
    > + tcon = tlink_tcon(cfile->tlink);
    > + server = tcon->ses->server;
    > +
    > + if (!server->ops->async_readv)
    > + return -ENOSYS;
    > +
    > + if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_RWPIDFORWARD)
    > + pid = cfile->pid;
    > + else
    > + pid = current->tgid;
    > +
    > + if ((file->f_flags & O_ACCMODE) == O_WRONLY)
    > + cifs_dbg(FYI, "attempting read on write only file instance\n");

    Confusing. Maybe "attempting read on write-only filehandle"?

    > +
    > + do {
    > + rc = server->ops->wait_mtu_credits(server, cifs_sb->rsize,
    > + &rsize, &credits);
    > + if (rc)
    > + break;
    > +
    > + cur_len = min_t(const size_t, len, rsize);
    > +
    > + rc = iov_iter_get_pages_alloc(to, &pagevec, cur_len, &start);
    > + if (rc < 0) {
    > + cifs_dbg(VFS,
    > + "couldn't get user pages (rc=%zd) iter type %d"
    > + " iov_offset %lu count %lu\n",
    > + rc, to->type, to->iov_offset, to->count);
    > + dump_stack();
    > + break;
    > + }
    > +
    > + rdata = cifs_readdata_direct_alloc(
    > + pagevec, cifs_direct_readv_complete);
    > + if (!rdata) {
    > + add_credits_and_wake_if(server, credits, 0);
    > + rc = -ENOMEM;
    > + break;
    > + }
    > +
    > + npages = (rc + start + PAGE_SIZE-1) / PAGE_SIZE;
    > + rdata->nr_pages = npages;
    > + rdata->page_offset = start;
    > + rdata->pagesz = PAGE_SIZE;
    > + rdata->tailsz = npages > 1 ?
    > + rc-(PAGE_SIZE-start)-(npages-2)*PAGE_SIZE :
    > + rc;

    This expression makes my head hurt. Surely it can be simplified, or
    expressed in a clearer way.

    > + cur_len = rc;
    > +
    > + rdata->cfile = cifsFileInfo_get(cfile);
    > + rdata->offset = offset;
    > + rdata->bytes = rc;
    > + rdata->pid = pid;
    > + rdata->read_into_pages = cifs_uncached_read_into_pages;
    > + rdata->copy_into_pages = cifs_uncached_copy_into_pages;
    > + rdata->credits = credits;
    > +
    > + rc = 0;
    > + if (rdata->cfile->invalidHandle)
    > + rc = cifs_reopen_file(rdata->cfile, true);
    > +
    > + if (!rc)
    > + rc = server->ops->async_readv(rdata);
    > +
    > + if (rc) {

    This whole rc thing is messy. Initializing to zero, setting only in
    one case, then testing the result, then setting it again, is twisted.
    I actually think a goto or two would read much more clearly.

    > + add_credits_and_wake_if(server, rdata->credits, 0);
    > + kref_put(&rdata->refcount,
    > + cifs_direct_readdata_release);
    > + if (rc == -EAGAIN)
    > + continue;
    > + break;

    It's worth a comment here that this either breaks or continues the
    entire do {} while (); and btw when it breaks it does *not* return "rc".
    Again, maybe a goto instead of a break?

    > + }
    > +
    > + wait_for_completion(&rdata->done);
    > + rc = rdata->result;
    > + if (rc) {
    > + kref_put(
    > + &rdata->refcount,
    > + cifs_direct_readdata_release);
    > + if (rc == -EAGAIN)
    > + continue;
    > + break;

    Ditto.

    > + }
    > +
    > + total_read += rdata->got_bytes;
    > + kref_put(&rdata->refcount, cifs_direct_readdata_release);
    > +
    > + iov_iter_advance(to, cur_len);
    > + len -= cur_len;
    > + offset += cur_len;
    > + } while (len);
    > +
    > + iocb->ki_pos += total_read;
    > +
    > + return total_read;
    > +}
    > +
    > ssize_t cifs_user_readv(struct kiocb *iocb, struct iov_iter *to)
    > {
    > struct file *file = iocb->ki_filp;
    >

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