Messages in this thread | | | Date | Fri, 8 Jun 2007 02:51:56 -0400 | From | "Albert Cahalan" <> | Subject | Re: [RFC][PATCH] /proc/pid/maps doesn't match "ipcs -m" shmid |
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On 6/8/07, Eric W. Biederman <ebiederm@xmission.com> wrote: > "Albert Cahalan" <acahalan@gmail.com> writes: > > On 6/7/07, Eric W. Biederman <ebiederm@xmission.com> wrote:
> >> So it looks to me like we need to do three things: > >> - Fix the inode number > >> - Fix the name on the hugetlbfs dentry to hold the key > >> - Add a big fat comment that user space programs depend on this > >> behavior of both the dentry name and the inode number. > > > > Assuming that this proposed fix goes in: > > > > Since the inode number is the shmid, and this is a number > > that the kernel randomly chooses AFAIK, there should be > > no need to have different shm segments sharing the same > > inode number. > > Where we run into inode number confusion is that all of > these shm segments are actually files on a tmpfs filesystem > somewhere, and by making the inode number the shmid we loose > the tmpfs inode number. So it is possible we get tmpfs inode > number conflicts. However the inode number is not used for > anything, and the files are not visible in any other way except > as shm segments so it doesn't matter.
Eh, the kernel choses both shmid and tmpfs inode number. You could set a high bit in one or the other.
> There is another case with ipc namespaces where we ultimately need > to support duplicate shmids on the same machine (so migration > is a possibility). However by and large the user space > processes with duplicate ids should be invisible to each other.
On the bright side, this only screws up people who get the crazy idea that processes can be migrated.
> > The situation with the key is a bit more disturbing, though > > we already hit that anyway when IPC_PRIVATE is used. > > (why anybody would NOT use IPC_PRIVATE is a mystery) > > So having the key in the name doesn't make things worse. > > Having "SYSV" in the name appears mandatory. Otherwise you > don't even know it is a shm file. Although I may be confused.
It's mandatory for a different reason: to satisfy parsers.
It is nearly useless for identifying shm files. Look what I can do: touch /SYSV00000000 touch '/SYSV00000000 (deleted)'
(so pmap creates a shm, looks for the address in /proc/self/maps, determines the device major/minor in use, and then uses that)
> Hmm. Thinking about this I have just realized that we may want > to approach this a little differently. Currently I am reusing > the dentry and inode structure that hugetlbfs and tmpfs return > me, and simply have a distinct struct file for each shm mapping. > > There is a little more cost but it may actually make sense to have > a dentry and inode that is specific to shm.c so we can do whatever > we need to without adding requirements to the normal tmpfs or hugtlb > code.
Piggybacking on tmpfs has always seemed a bit dirty to me. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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