From: Vivek Goyal <vgoyal@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
Stefan Hajnoczi <stefanha@redhat.com>
Cc: virtio-fs-list <virtio-fs@redhat.com>
Subject: [Virtio-fs] Few queries about virtiofsd read implementation
Date: Fri, 7 May 2021 14:59:09 -0400 [thread overview]
Message-ID: <20210507185909.GA40951@redhat.com> (raw)
Hi David/Stefan,
I am browsing through the code of read requests (FUSE_READ) in virtiofsd
(and in virtiofs) and I have few questions. You folks probably know the
answers.
1. virtio_send_data_iov(), reads the data from file into the scatter list.
Some of the code looks strange.
We seem to be retrying read if we read less number of bytes than what
client asked for. I am wondering shoudl this really be our
responsibility or client should deal with it. I am assuming that client
should be ready to deal with less number of bytes read.
So what was the thought process behind retrying.
if (ret < len && ret) {
fuse_log(FUSE_LOG_DEBUG, "%s: ret < len\n", __func__);
/* Skip over this much next time around */
skip_size = ret;
buf->buf[0].pos += ret;
len -= ret;
/* Lets do another read */
continue;
}
- After this we have code where if number of bytes read are not same
as we expect to, then we return EIO.
if (ret != len) {
fuse_log(FUSE_LOG_DEBUG, "%s: ret!=len\n", __func__);
ret = EIO;
free(in_sg_cpy);
goto err;
}
When do we hit this. IIUC, preadv() will return.
A. Either number of bytes we expected (no issues)
B. 0 in case of EOF (We break out of loop and just return to client with
number of bytes we have read so far).
C. <0 (This is error case and we return error to client)
D. X bytes which is less than len.
To handle D we have code to retry. So when do we hit the above if
condition where "ret !=len). Is this a dead code. Or I missed something.
2. When client sent FUSE_READ, we put pointer to pages into sglist. IIUC,
we put pointer to "struct page *" and not actual page. So who converts
these struct page pointer to memory belong to page.
sg_init_fuse_pages() {
sg_set_page(&sg[i], pages[i], this_len, page_descs[i].offset);
}
3. Who converts guest memory address (when) into qemu process address which is
accessible by virtiofsd.
Thanks
Vivek
next reply other threads:[~2021-05-07 18:59 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-07 18:59 Vivek Goyal [this message]
2021-05-07 21:14 ` [Virtio-fs] Few queries about virtiofsd read implementation Edward McClanahan
2021-05-10 13:10 ` Vivek Goyal
2021-05-10 17:23 ` Dr. David Alan Gilbert
2021-05-10 21:40 ` Vivek Goyal
2021-05-10 21:45 ` Vivek Goyal
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