[Virtio-fs] [PATCH v2 2/7] virtiofds: Changed allocations of iovec to GLib's functions
Mahmoud Mandour
ma.mandourr at gmail.com
Tue Apr 27 11:08:39 UTC 2021
On Tue, Apr 27, 2021 at 1:01 PM Dr. David Alan Gilbert <dgilbert at redhat.com>
wrote:
> * Mahmoud Mandour (ma.mandourr at gmail.com) wrote:
> > On Tue, Apr 27, 2021 at 12:25 PM Dr. David Alan Gilbert <
> dgilbert at redhat.com>
> > wrote:
> >
> > > * Mahmoud Mandour (ma.mandourr at gmail.com) wrote:
> > > > Replaced the calls to malloc()/calloc() and their respective
> > > > calls to free() of iovec structs with GLib's allocation and
> > > > deallocation functions.
> > > >
> > > > Also, in one instance, used g_new0() instead of a calloc() call plus
> > > > a null-checking assertion.
> > > >
> > > > iovec structs were created locally and freed as the function
> > > > ends. Hence, I used g_autofree and removed the respective calls to
> > > > free().
> > > >
> > > > In one instance, a struct fuse_ioctl_iovec pointer is returned from a
> > > > function, namely, fuse_ioctl_iovec_copy. There, I used
> g_steal_pointer()
> > > > in conjunction with g_autofree, this gives the ownership of the
> pointer
> > > > to the calling function and still auto-frees the memory when the
> calling
> > > > function finishes (maintaining the symantics of previous code).
> > > >
> > > > Signed-off-by: Mahmoud Mandour <ma.mandourr at gmail.com>
> > > > Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> > > > ---
> > > > tools/virtiofsd/fuse_lowlevel.c | 19 +++++++------------
> > > > tools/virtiofsd/fuse_virtio.c | 6 +-----
> > > > 2 files changed, 8 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/tools/virtiofsd/fuse_lowlevel.c
> > > b/tools/virtiofsd/fuse_lowlevel.c
> > > > index 812cef6ef6..f965299ad9 100644
> > > > --- a/tools/virtiofsd/fuse_lowlevel.c
> > > > +++ b/tools/virtiofsd/fuse_lowlevel.c
> > > > @@ -217,9 +217,9 @@ static int send_reply(fuse_req_t req, int error,
> > > const void *arg,
> > > > int fuse_reply_iov(fuse_req_t req, const struct iovec *iov, int
> count)
> > > > {
> > > > int res;
> > > > - struct iovec *padded_iov;
> > > > + g_autofree struct iovec *padded_iov;
> > > >
> > > > - padded_iov = malloc((count + 1) * sizeof(struct iovec));
> > > > + padded_iov = g_try_new(struct iovec, count + 1);
> > > > if (padded_iov == NULL) {
> > > > return fuse_reply_err(req, ENOMEM);
> > > > }
> > > > @@ -228,7 +228,6 @@ int fuse_reply_iov(fuse_req_t req, const struct
> > > iovec *iov, int count)
> > > > count++;
> > > >
> > > > res = send_reply_iov(req, 0, padded_iov, count);
> > > > - free(padded_iov);
> > > >
> > > > return res;
> > > > }
> > >
> > > OK.
> > >
> > > > @@ -565,10 +564,10 @@ int fuse_reply_bmap(fuse_req_t req, uint64_t
> idx)
> > > > static struct fuse_ioctl_iovec *fuse_ioctl_iovec_copy(const struct
> > > iovec *iov,
> > > > size_t count)
> > > > {
> > > > - struct fuse_ioctl_iovec *fiov;
> > > > + g_autofree struct fuse_ioctl_iovec *fiov;
> > > > size_t i;
> > > >
> > > > - fiov = malloc(sizeof(fiov[0]) * count);
> > > > + fiov = g_try_new(fuse_ioctl_iovec, count);
> > > > if (!fiov) {
> > > > return NULL;
> > > > }
> > > > @@ -578,7 +577,7 @@ static struct fuse_ioctl_iovec
> > > *fuse_ioctl_iovec_copy(const struct iovec *iov,
> > > > fiov[i].len = iov[i].iov_len;
> > > > }
> > > >
> > > > - return fiov;
> > > > + return g_steal_pointer(&fiov);
> > > > }
> > >
> > > This is OK, but doesn't gain anything - marking it as g_autofree'ing
> and
> > > always stealing is no benefit.
> > >
> > > >
> > > > int fuse_reply_ioctl_retry(fuse_req_t req, const struct iovec
> *in_iov,
> > > > @@ -629,9 +628,6 @@ int fuse_reply_ioctl_retry(fuse_req_t req, const
> > > struct iovec *in_iov,
> > > >
> > > > res = send_reply_iov(req, 0, iov, count);
> > > > out:
> > > > - free(in_fiov);
> > > > - free(out_fiov);
> > > > -
> > >
> > > I don't think you can do that - I think you're relying here on the
> > > g_autofree from fuse_ioclt_iovec_copy - but my understanding is that
> > > doesn't work; g_autofree is scoped, so it's designed to free at the end
> > > of fuse_ioctl_iovec_copy, fuse_reply_ioctl_retry doesn't know that the
> > > ion_fiov were allocated that way, so it won't get autocleaned up.
> > >
> > >
> > In GLib's documentation, it is clarified (w.r.t. g_autoptr but I think
> > similar logic applies to g_autofree)
> > that g_steal_pointer() "This can be very useful when combined with
> > g_autoptr() to prevent
> > the return value of a function from being automatically freed."
> > I think, but not 100% clear of course, that this means that the
> > g_autoptr-annotated memory
> > does not get freed at the end of the current scope, and its "scope" is
> > migrated to the calling
> > function(to be honest I don't know how would they implement that but
> maybe
> > this is the case).
> > Otherwise why bother with g_autoptr'ing memory that we don't want to free
> > automatically and
> > would like to return to the calling function?
> >
> > The first example in Memory Allocation: GLib Reference Manual (gnome.org
> )
> > <
> https://developer.gnome.org/glib/stable/glib-Memory-Allocation.html#g-steal-pointer
> >
> > does
> > annotate
> > the memory as g_autoptr and then returns it through g_steal_pointer. With
> > your logic, I think that
> > this example would be wrong(?)
>
> The example is correct but not quite the case you have; the
> g_steal_pointer stops the g_autoptr freeing it at the end of the current
> scope; but it doesn't cause it to be free'd later - the caller can't
> tell that the function that did the allocation had a g_autofree in it;
> once you get outside of the function, the pointer is just a normal
> pointer that needs free or g_free on.
>
> I think that this is logical, yes. I think that I understand now. Can you
please instruct
me on what to do with the patch? Do you want me to resend the entire patch
series
and amend this one?
>
> > Mr. Hajnoczi already reviewed this patch Re: [PATCH 2/8] virtiofds:
> > Changed allocations of iovec to GLib's functi
> > <https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg08459.html>
> > in a previous version and this v2 patch series is supposed to only
> contain
> > already-reviewed patches and
> > remove bad ones
>
> But he didn't spot this particular problem.
>
> Dave
>
> >
> > > > return res;
> > > >
> > > > enomem:
> > > > @@ -663,11 +659,11 @@ int fuse_reply_ioctl(fuse_req_t req, int
> result,
> > > const void *buf, size_t size)
> > > > int fuse_reply_ioctl_iov(fuse_req_t req, int result, const struct
> iovec
> > > *iov,
> > > > int count)
> > > > {
> > > > - struct iovec *padded_iov;
> > > > + g_autofree struct iovec *padded_iov;
> > > > struct fuse_ioctl_out arg;
> > > > int res;
> > > >
> > > > - padded_iov = malloc((count + 2) * sizeof(struct iovec));
> > > > + padded_iov = g_try_new(struct iovec, count + 2);
> > > > if (padded_iov == NULL) {
> > > > return fuse_reply_err(req, ENOMEM);
> > > > }
> > > > @@ -680,7 +676,6 @@ int fuse_reply_ioctl_iov(fuse_req_t req, int
> result,
> > > const struct iovec *iov,
> > > > memcpy(&padded_iov[2], iov, count * sizeof(struct iovec));
> > > >
> > > > res = send_reply_iov(req, 0, padded_iov, count + 2);
> > > > - free(padded_iov);
> > > >
> > > > return res;
> > > > }
> > >
> > > OK
> > >
> > > > diff --git a/tools/virtiofsd/fuse_virtio.c
> > > b/tools/virtiofsd/fuse_virtio.c
> > > > index 3e13997406..07e5d91a9f 100644
> > > > --- a/tools/virtiofsd/fuse_virtio.c
> > > > +++ b/tools/virtiofsd/fuse_virtio.c
> > > > @@ -347,8 +347,7 @@ int virtio_send_data_iov(struct fuse_session *se,
> > > struct fuse_chan *ch,
> > > > * Build a copy of the the in_sg iov so we can skip bits in it,
> > > > * including changing the offsets
> > > > */
> > > > - struct iovec *in_sg_cpy = calloc(sizeof(struct iovec), in_num);
> > > > - assert(in_sg_cpy);
> > > > + g_autofree struct iovec *in_sg_cpy = g_new0(struct iovec,
> in_num);
> > > > memcpy(in_sg_cpy, in_sg, sizeof(struct iovec) * in_num);
> > > > /* These get updated as we skip */
> > > > struct iovec *in_sg_ptr = in_sg_cpy;
> > > > @@ -386,7 +385,6 @@ int virtio_send_data_iov(struct fuse_session *se,
> > > struct fuse_chan *ch,
> > > > ret = errno;
> > > > fuse_log(FUSE_LOG_DEBUG, "%s: preadv failed (%m)
> len=%zd\n",
> > > > __func__, len);
> > > > - free(in_sg_cpy);
> > > > goto err;
> > > > }
> > > > fuse_log(FUSE_LOG_DEBUG, "%s: preadv ret=%d len=%zd\n",
> > > __func__,
> > > > @@ -410,13 +408,11 @@ int virtio_send_data_iov(struct fuse_session
> *se,
> > > struct fuse_chan *ch,
> > > > if (ret != len) {
> > > > fuse_log(FUSE_LOG_DEBUG, "%s: ret!=len\n", __func__);
> > > > ret = EIO;
> > > > - free(in_sg_cpy);
> > > > goto err;
> > > > }
> > > > in_sg_left -= ret;
> > > > len -= ret;
> > > > } while (in_sg_left);
> > > > - free(in_sg_cpy);
> > >
> > > Yes, this is where the autofree really helps; getting rid of a few
> > > free's.
> > >
> > > Dave
> > >
> > > > /* Need to fix out->len on EOF */
> > > > if (len) {
> > > > --
> > > > 2.25.1
> > > >
> > > --
> > > Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
> > >
> > >
> > Thanks,
> > Mahmoud
> --
> Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
>
>
Thanks,
Mahmoud
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