[PATCH xwayland] xwayland-shm: fortify fallocate against EINTR

[Pekka Paalanen]

On Mon, 25 Apr 2016 21:07:17 +0800
Jonas Ådahl <jadahl at gmail.com> wrote:

> On Mon, Apr 25, 2016 at 02:38:21PM +0200, Marek Chalupa wrote:
> > 
> > On 04/25/16 12:20, Pekka Paalanen wrote:  
> > >On Mon, 25 Apr 2016 11:33:00 +0200
> > >Marek Chalupa <mchqwerty at gmail.com> wrote:
> > >  
> > >>If posix_fallocate or ftruncate is interrupted by signal
> > >>while working, we return -1 as fd and the allocation process
> > >>returns BadAlloc error. That causes xwayland clients to abort
> > >>with 'BadAlloc (insufficient resources for operation)'
> > >>even when there's a lot of resources available.
> > >>
> > >>Fix it by trying again when we get EINTR.
> > >>
> > >>Signed-off-by: Marek Chalupa <mchqwerty at gmail.com>
> > >>---
> > >>  hw/xwayland/xwayland-shm.c | 10 ++++++++--
> > >>  1 file changed, 8 insertions(+), 2 deletions(-)
> > >>
> > >>diff --git a/hw/xwayland/xwayland-shm.c b/hw/xwayland/xwayland-shm.c
> > >>index e8545b3..c199e5e 100644
> > >>--- a/hw/xwayland/xwayland-shm.c
> > >>+++ b/hw/xwayland/xwayland-shm.c
> > >>@@ -140,14 +140,20 @@ os_create_anonymous_file(off_t size)
> > >>          return -1;
> > >>
> > >>  #ifdef HAVE_POSIX_FALLOCATE
> > >>-    ret = posix_fallocate(fd, 0, size);
> > >>+    do {
> > >>+        ret = posix_fallocate(fd, 0, size);
> > >>+    } while (ret == EINTR);
> > >>+
> > >>      if (ret != 0) {
> > >>          close(fd);
> > >>          errno = ret;
> > >>          return -1;
> > >>      }
> > >>  #else
> > >>-    ret = ftruncate(fd, size);
> > >>+    do {
> > >>+        ret = ftruncate(fd, size);
> > >>+    } while (ret == -1 && errno == EINTR);
> > >>+
> > >>      if (ret < 0) {
> > >>          close(fd);
> > >>          return -1;  
> > >
> > >Hi Marek,
> > >
> > >curious, how did you hit this case? And is the signal that intercept
> > >these usually the smart scheduler's SIGALRM?  
> > 
> > Hi Pekka,
> > 
> > under gnome-shell it's enough to open terminator and resize it very
> > quickly. Under Weston (and also gnome-shell) I hit this with Firefox.
> > Just start it and in a moment it goes down with this error.
> > 
> > Yes, it looks like SIGALRM (I put assert on change of SmartScheduleTime
> > when fallocate fails)
> >   
> > >I am asking, because I have someone suffering from the EINTR issue, but
> > >a simple restart like what you implemented here results in an endless
> > >loop. A new signal arrives before fallocate completes every time. It is
> > >like fallocate is not making any progress.
> > >
> > >What is more curious is that the file is supposedly on a tmpfs, yet in
> > >our case the 5 ms is not enough to fallocate a full-HD frame (8 MB). It
> > >is a "Low powered NXP arm platform" I am told, I do not have access to
> > >it myself.  
> 
> Couldn't it just as well be that the allocation started 0.01 ms before
> the SIGALRM that interrupted it?

If it happened only the first time, sure. But I am told Xwayland
actually gets into an endless loop with the particular fallocate never
returning anything but EINTR all the time.


Thanks,
pq

> > >
> > >It may be the platform's fault that fallocate takes such a long time.
> > >Another thing is whether fallocate should make gradual progress or not;
> > >if not, simple restart will not work against a regular timer signal.  
> > 
> > Here the platform is x86_64. Actually, this started happening some time
> > ago, but it was fine before. I don't know what made the change.  
> 
> Not too long ago the posix_fallocate() support was fixed in Xwayland.
> Some configure.ac rules were missing and we were simply never ever
> building those lines behind the #ifdef. I assume this is what made this
> issue suddenly show up.
> 
> 
> Jonas
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