> Subject: Re: [Patch v4 1/3] CIFS: Add support for direct I/O read
>
> Hi Long,
>
> Please find my comments below.
>
>
> ср, 31 окт. 2018 г. в 15:14, Long Li <longli@xxxxxxxxxxxxxxxxx>:
> >
> > From: Long Li <longli@xxxxxxxxxxxxx>
> >
> > With direct I/O read, we transfer the data directly from transport
> > layer to the user data buffer.
> >
> > Change in v3: add support for kernel AIO
> >
> > Change in v4:
> > Refactor common read code to __cifs_readv for direct and non-direct I/O.
> > Retry on direct I/O failure.
> >
> > Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
> > ---
> > fs/cifs/cifsfs.h | 1 +
> > fs/cifs/cifsglob.h | 5 ++
> > fs/cifs/file.c | 219 +++++++++++++++++++++++++++++++++++++++++++--
> --------
> > 3 files changed, 186 insertions(+), 39 deletions(-)
> >
> > 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/cifsglob.h
> > b/fs/cifs/cifsglob.h index 7f62c98..52248dd 100644
> > --- a/fs/cifs/cifsglob.h
> > +++ b/fs/cifs/cifsglob.h
> > @@ -1146,6 +1146,11 @@ struct cifs_aio_ctx {
> > unsigned int len;
> > unsigned int total_len;
> > bool should_dirty;
> > + /*
> > + * Indicates if this aio_ctx is for direct_io,
> > + * If yes, iter is a copy of the user passed iov_iter
> > + */
> > + bool direct_io;
> > };
> >
> > struct cifs_readdata;
> > diff --git a/fs/cifs/file.c b/fs/cifs/file.c index 87eece6..daab878
> > 100644
> > --- a/fs/cifs/file.c
> > +++ b/fs/cifs/file.c
> > @@ -2965,7 +2965,6 @@ cifs_uncached_readdata_release(struct kref
> *refcount)
> > kref_put(&rdata->ctx->refcount, cifs_aio_ctx_release);
> > for (i = 0; i < rdata->nr_pages; i++) {
> > put_page(rdata->pages[i]);
> > - rdata->pages[i] = NULL;
> > }
> > cifs_readdata_release(refcount); } @@ -3092,6 +3091,63 @@
> > cifs_uncached_copy_into_pages(struct TCP_Server_Info *server,
> > return uncached_fill_pages(server, rdata, iter, iter->count);
> > }
> >
> > +static int cifs_resend_rdata(struct cifs_readdata *rdata,
> > + struct list_head *rdata_list,
> > + struct cifs_aio_ctx *ctx) {
> > + int wait_retry = 0;
> > + unsigned int rsize, credits;
> > + int rc;
> > + struct TCP_Server_Info *server =
> > +tlink_tcon(rdata->cfile->tlink)->ses->server;
> > +
> > + /*
> > + * Try to resend this rdata, waiting for credits up to 3 seconds.
> > + * Note: we are attempting to resend the whole rdata not in segments
> > + */
> > + do {
> > + rc = server->ops->wait_mtu_credits(server, rdata->bytes,
> > + &rsize, &credits);
> > +
> > + if (rc)
> > + break;
> > +
> > + if (rsize < rdata->bytes) {
> > + add_credits_and_wake_if(server, credits, 0);
> > + msleep(1000);
> > + wait_retry++;
> > + }
> > + } while (rsize < rdata->bytes && wait_retry < 3);
> > +
> > + /*
> > + * If we can't find enough credits to send this rdata
> > + * release the rdata and return failure, this will pass
> > + * whatever I/O amount we have finished to VFS.
> > + */
> > + if (rsize < rdata->bytes) {
> > + rc = -EBUSY;
>
> We don't have enough credits and return EBUSY here...
>
> > + goto out;
> > + }
> > +
> > + rc = -EAGAIN;
> > + while (rc == -EAGAIN)
> > + if (!rdata->cfile->invalidHandle ||
> > + !(rc = cifs_reopen_file(rdata->cfile, true)))
> > + rc = server->ops->async_readv(rdata);
> > +
> > + if (!rc) {
> > + /* Add to aio pending list */
> > + list_add_tail(&rdata->list, rdata_list);
> > + return 0;
> > + }
> > +
> > + add_credits_and_wake_if(server, rdata->credits, 0);
> > +out:
> > + kref_put(&rdata->refcount,
> > + cifs_uncached_readdata_release);
> > +
> > + return rc;
> > +}
> > +
> > static int
> > cifs_send_async_read(loff_t offset, size_t len, struct cifsFileInfo *open_file,
> > struct cifs_sb_info *cifs_sb, struct list_head
> > *rdata_list, @@ -3103,6 +3159,9 @@ cifs_send_async_read(loff_t offset,
> size_t len, struct cifsFileInfo *open_file,
> > int rc;
> > pid_t pid;
> > struct TCP_Server_Info *server;
> > + struct page **pagevec;
> > + size_t start;
> > + struct iov_iter direct_iov = ctx->iter;
> >
> > server = tlink_tcon(open_file->tlink)->ses->server;
> >
> > @@ -3111,6 +3170,9 @@ cifs_send_async_read(loff_t offset, size_t len,
> struct cifsFileInfo *open_file,
> > else
> > pid = current->tgid;
> >
> > + if (ctx->direct_io)
> > + iov_iter_advance(&direct_iov, offset - ctx->pos);
> > +
> > do {
> > rc = server->ops->wait_mtu_credits(server, cifs_sb->rsize,
> > &rsize, &credits);
> > @@ -3118,20 +3180,56 @@ cifs_send_async_read(loff_t offset, size_t len,
> struct cifsFileInfo *open_file,
> > break;
> >
> > cur_len = min_t(const size_t, len, rsize);
> > - npages = DIV_ROUND_UP(cur_len, PAGE_SIZE);
> >
> > - /* allocate a readdata struct */
> > - rdata = cifs_readdata_alloc(npages,
> > + if (ctx->direct_io) {
> > +
> > + cur_len = iov_iter_get_pages_alloc(
> > + &direct_iov, &pagevec,
> > + cur_len, &start);
> > + if (cur_len < 0) {
> > + cifs_dbg(VFS,
> > + "couldn't get user pages (cur_len=%zd)"
> > + " iter type %d"
> > + " iov_offset %zd count %zd\n",
> > + cur_len, direct_iov.type, direct_iov.iov_offset,
> > + direct_iov.count);
> > + dump_stack();
> > + break;
> > + }
> > + iov_iter_advance(&direct_iov, cur_len);
> > +
> > + rdata = cifs_readdata_direct_alloc(
> > + pagevec, cifs_uncached_readv_complete);
> > + if (!rdata) {
> > + add_credits_and_wake_if(server, credits, 0);
> > + rc = -ENOMEM;
> > + break;
> > + }
> > +
> > + npages = (cur_len + start + PAGE_SIZE-1) / PAGE_SIZE;
> > + rdata->page_offset = start;
> > + rdata->tailsz = npages > 1 ?
> > + cur_len-(PAGE_SIZE-start)-(npages-2)*PAGE_SIZE :
> > + cur_len;
> > +
> > + } else {
> > +
> > + npages = DIV_ROUND_UP(cur_len, PAGE_SIZE);
> > + /* allocate a readdata struct */
> > + rdata = cifs_readdata_alloc(npages,
> > cifs_uncached_readv_complete);
> > - if (!rdata) {
> > - add_credits_and_wake_if(server, credits, 0);
> > - rc = -ENOMEM;
> > - break;
> > - }
> > + if (!rdata) {
> > + add_credits_and_wake_if(server, credits, 0);
> > + rc = -ENOMEM;
> > + break;
> > + }
> >
> > - rc = cifs_read_allocate_pages(rdata, npages);
> > - if (rc)
> > - goto error;
> > + rc = cifs_read_allocate_pages(rdata, npages);
> > + if (rc)
> > + goto error;
> > +
> > + rdata->tailsz = PAGE_SIZE;
> > + }
> >
> > rdata->cfile = cifsFileInfo_get(open_file);
> > rdata->nr_pages = npages; @@ -3139,7 +3237,6 @@
> > cifs_send_async_read(loff_t offset, size_t len, struct cifsFileInfo *open_file,
> > rdata->bytes = cur_len;
> > rdata->pid = pid;
> > rdata->pagesz = PAGE_SIZE;
> > - rdata->tailsz = PAGE_SIZE;
> > rdata->read_into_pages = cifs_uncached_read_into_pages;
> > rdata->copy_into_pages = cifs_uncached_copy_into_pages;
> > rdata->credits = credits; @@ -3153,9 +3250,11 @@
> > cifs_send_async_read(loff_t offset, size_t len, struct cifsFileInfo *open_file,
> > if (rc) {
> > add_credits_and_wake_if(server, rdata->credits, 0);
> > kref_put(&rdata->refcount,
> > - cifs_uncached_readdata_release);
> > - if (rc == -EAGAIN)
> > + cifs_uncached_readdata_release);
> > + if (rc == -EAGAIN) {
> > + iov_iter_revert(&direct_iov, cur_len);
> > continue;
> > + }
> > break;
> > }
> >
> > @@ -3211,45 +3310,62 @@ collect_uncached_read_data(struct
> cifs_aio_ctx *ctx)
> > * reading.
> > */
> > if (got_bytes && got_bytes < rdata->bytes) {
> > - rc = cifs_readdata_to_iov(rdata, to);
> > + rc = 0;
> > + if (!ctx->direct_io)
> > + rc =
> > + cifs_readdata_to_iov(rdata, to);
> > if (rc) {
> > kref_put(&rdata->refcount,
> > - cifs_uncached_readdata_release);
> > +
> > + cifs_uncached_readdata_release);
> > continue;
> > }
> > }
> >
> > - rc = cifs_send_async_read(
> > + if (ctx->direct_io) {
> > + /*
> > + * Re-use rdata as this is a
> > + * direct I/O
> > + */
> > + rc = cifs_resend_rdata(
> > + rdata,
> > + &tmp_list, ctx);
>
> ...so we set rc to -EBUSY...
>
>
> > + } else {
> > + rc = cifs_send_async_read(
> > rdata->offset + got_bytes,
> > rdata->bytes - got_bytes,
> > rdata->cfile, cifs_sb,
> > &tmp_list, ctx);
> >
> > + kref_put(&rdata->refcount,
> > + cifs_uncached_readdata_release);
> > + }
> > +
> > list_splice(&tmp_list, &ctx->list);
> >
> > - kref_put(&rdata->refcount,
> > - cifs_uncached_readdata_release);
> > goto again;
> > } else if (rdata->result)
> > rc = rdata->result;
> > - else
> > + else if (!ctx->direct_io)
> > rc = cifs_readdata_to_iov(rdata, to);
> >
> > /* if there was a short read -- discard anything left */
> > if (rdata->got_bytes && rdata->got_bytes < rdata->bytes)
> > rc = -ENODATA;
> > +
> > + ctx->total_len += rdata->got_bytes;
> > }
> > list_del_init(&rdata->list);
> > kref_put(&rdata->refcount, cifs_uncached_readdata_release);
> > }
> >
> > - for (i = 0; i < ctx->npages; i++) {
> > - if (ctx->should_dirty)
> > - set_page_dirty(ctx->bv[i].bv_page);
> > - put_page(ctx->bv[i].bv_page);
> > - }
> > + if (!ctx->direct_io) {
> > + for (i = 0; i < ctx->npages; i++) {
> > + if (ctx->should_dirty)
> > + set_page_dirty(ctx->bv[i].bv_page);
> > + put_page(ctx->bv[i].bv_page);
> > + }
> >
> > - ctx->total_len = ctx->len - iov_iter_count(to);
> > + ctx->total_len = ctx->len - iov_iter_count(to);
> > + }
> >
> > cifs_stats_bytes_read(tcon, ctx->total_len);
> >
> > @@ -3267,18 +3383,27 @@ collect_uncached_read_data(struct
> cifs_aio_ctx *ctx)
> > complete(&ctx->done);
>
> and then ctx->rc is set to -EBUSY too.
>
> > }
> >
> > -ssize_t cifs_user_readv(struct kiocb *iocb, struct iov_iter *to)
> > +ssize_t __cifs_readv(struct kiocb *iocb, struct iov_iter *to, bool
> > +direct)
> > {
> > - struct file *file = iocb->ki_filp;
> > - ssize_t rc;
> > size_t len;
> > - ssize_t total_read = 0;
> > - loff_t offset = iocb->ki_pos;
> > + struct file *file = iocb->ki_filp;
> > struct cifs_sb_info *cifs_sb;
> > - struct cifs_tcon *tcon;
> > struct cifsFileInfo *cfile;
> > + struct cifs_tcon *tcon;
> > + ssize_t rc, total_read = 0;
> > + loff_t offset = iocb->ki_pos;
> > struct cifs_aio_ctx *ctx;
> >
> > + /*
> > + * iov_iter_get_pages_alloc() doesn't work with ITER_KVEC,
> > + * fall back to data copy read path
> > + * this could be improved by getting pages directly in ITER_KVEC
> > + */
> > + if (direct && to->type & ITER_KVEC) {
> > + cifs_dbg(FYI, "use non-direct cifs_user_readv for kvec I/O\n");
> > + direct = false;
> > + }
> > +
> > len = iov_iter_count(to);
> > if (!len)
> > return 0;
> > @@ -3305,14 +3430,20 @@ ssize_t cifs_user_readv(struct kiocb *iocb,
> struct iov_iter *to)
> > if (to->type == ITER_IOVEC)
> > ctx->should_dirty = true;
> >
> > - rc = setup_aio_ctx_iter(ctx, to, READ);
> > - if (rc) {
> > - kref_put(&ctx->refcount, cifs_aio_ctx_release);
> > - return rc;
> > + if (direct) {
> > + ctx->pos = offset;
> > + ctx->direct_io = true;
> > + ctx->iter = *to;
> > + ctx->len = len;
> > + } else {
> > + rc = setup_aio_ctx_iter(ctx, to, READ);
> > + if (rc) {
> > + kref_put(&ctx->refcount, cifs_aio_ctx_release);
> > + return rc;
> > + }
> > + len = ctx->len;
> > }
> >
> > - len = ctx->len;
> > -
> > /* grab a lock here due to read response handlers can access ctx */
> > mutex_lock(&ctx->aio_mutex);
> >
> > @@ -3354,6 +3485,16 @@ ssize_t cifs_user_readv(struct kiocb *iocb,
> struct iov_iter *to)
> > return rc;
>
> In case of a blocking read we will return -EBUSY to the caller but read man
> page doesn't say anything about a possibility to return this error code. Is it
> being mapped somehow by VFS or is there another consideration? The same
> question applies to the write patch as well.
Thanks Pavel, Yes this is a bug.
In practice, the retry logic will rarely get hit. If getting hit, normally the server will give us far more credits for resending this whole packet.
How about just retrying forever (instead of trying 3 times) on this resend packet? This will change the code to the same behavior before direct I/O patches.
e.g.:
/*
* Try to resend this wdata, waiting for credits before sending
* Note: we are attempting to resend the whole wdata not in segments
*/
do {
rc = server->ops->wait_mtu_credits(
server, wdata->bytes, &wsize, &credits);
if (rc)
break;
if (wsize < wdata->bytes) {
add_credits_and_wake_if(server, credits, 0);
msleep(1000);
}
} while (wsize < wdata->bytes);
>
> > }
> >
> > +ssize_t cifs_direct_readv(struct kiocb *iocb, struct iov_iter *to) {
> > + return __cifs_readv(iocb, to, true); }
> > +
> > +ssize_t cifs_user_readv(struct kiocb *iocb, struct iov_iter *to) {
> > + return __cifs_readv(iocb, to, false); }
> > +
> > ssize_t
> > cifs_strict_readv(struct kiocb *iocb, struct iov_iter *to) {
> > --
> > 2.7.4
> >
>
> --
> Best regards,
> Pavel Shilovsky
