On Thu, 5 Jul 2012 19:27:48 +0200 Piergiorgio Sartor <piergiorgio.sartor@xxxxxxxx> wrote: > Hi all, > > following the comment of Neil about the fact that > an md device, in r/o mode, will BUG() in case of > write attempt, I would like to ask for some more > information on the topic. > > The problem is the following, mounting a filesystem > in r/o does not, correctly, prevent writes to the > underlying device. This is only half the story. If the mount command requests read-only, the filesystem may still write to the device. However if the device is marked read-only then the filesystem should not write to the disk - if it finds that it needs to, it should fail the mount request. I believe that filesystems behave correctly in this manner, though there could still be bugs lurking. > > Now, assuming a filesystem is mounted r/w over an > md device, being r/w too, it is pretty clear that > switching the md device to r/o will create problems. Not quite. When the filesystem is mounted, md will refuse to switch the device to read-only, so that scenario is not possible. > > What if the md device is started in r/o mode and > than, later, the filesystem is mounted *without* > specifying r/o mode? The mount system call will notice that the device is read-only, and will make assume the read-only mount option. > > The filesystem might try to write to the device, > but it will not work, of course. > Does this will cause a BUG() in md? Only if there are bugs. > > Because using a r/o memory card, there are no > problems, the mount recognize the device is r/o > and it sets the filesystems r/o too. > I do not know if it anyway tries to write to the > r/o device, ignoring the errors reported. > > I would say that, if md BUG() in any case, it > could be a bug... :-) > > Any comments? > > Thanks, > > bye, > NeilBrown
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