On 8 July 2015 at 21:17, Doug Ledford <dledford@xxxxxxxxxx> wrote: > On 07/07/2015 09:01 AM, Michael Kerrisk (man-pages) wrote: >> Hi David, >> >> On 7 July 2015 at 07:16, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote: >>> On Mon, 2015-07-06 at 17:49 +0200, Marcus Gelderie wrote: >>>> A while back, the message queue implementation in the kernel was >>>> improved to use btrees to speed up retrieval of messages (commit >>>> d6629859b36). The patch introducing the improved kernel handling of >>>> message queues (using btrees) has, as a by-product, changed the >>>> meaning of the QSIZE field in the pseudo-file created for the queue. >>>> Before, this field reflected the size of the user-data in the queue. >>>> Since, it also takes kernel data structures into account. For >>>> example, if 13 bytes of user data are in the queue, on my machine the >>>> file reports a size of 61 bytes. >>>> >>>> There was some discussion on this topic before (for example >>>> https://lkml.org/lkml/2014/10/1/115). Commenting on a th lkml, Michael >>>> Kerrisk gave the following background (https://lkml.org/lkml/2015/6/16/74): >>>> >>>> The pseudofiles in the mqueue filesystem (usually mounted at >>>> /dev/mqueue) expose fields with metadata describing a message >>>> queue. One of these fields, QSIZE, as originally implemented, >>>> showed the total number of bytes of user data in all messages in >>>> the message queue, and this feature was documented from the >>>> beginning in the mq_overview(7) page. In 3.5, some other (useful) >>>> work happened to break the user-space API in a couple of places, >>>> including the value exposed via QSIZE, which now includes a measure >>>> of kernel overhead bytes for the queue, a figure that renders QSIZE >>>> useless for its original purpose, since there's no way to deduce >>>> the number of overhead bytes consumed by the implementation. >>>> (The other user-space breakage was subsequently fixed.) >>> >>> Michael, this breakage was never finally documented in the manpage, >>> right? >> >> Right. >> >>> I took a look and there is no mention, but it was a quick look. >>> It's just that if this patch goes in, I'd hate ending up with something >>> like this in the manpage: >>> >>> as of 3.5 >>> <accounts for kernel overhead> >>> >>> as of 4.3 >>> <behavior reverted back to not include kernel overhead... *sigh*> >>> >>> If there are changes to be made to the manpage, it should probably be >>> posted with this patch, methinks. >> >> I think something like the above will have to end up in the man page. >> The only thing I'd add is that the fix also has gone to -stable >> kernels. At least: I think this patch should also be marked for >> -stable. I'll take care of the man page updates as the patch goes >> through. >> >>>> This patch removes the accounting of kernel data structures in the >>>> queue. Reporting the size of these data-structures in the QSIZE field >>>> was a breaking change (see Michael's comment above). Without the QSIZE >>>> field reporting the total size of user-data in the queue, there is no >>>> way to deduce this number. >>>> >>>> It should be noted that the resource limit RLIMIT_MSGQUEUE is counted >>>> against the worst-case size of the queue (in both the old and the new >>>> implementation). Therefore, the kernel overhead accounting in QSIZE is >>>> not necessary to help the user understand the limitations RLIMIT imposes >>>> on the processes. >>> >>> Also, I would suggest adding some comment in struct mqueue_inode_info >>> for future reference, ie: >>> >>> - unsigned long qsize; /* size of queue in memory (sum of all msgs) */ >>> + /* >>> + * Size of queue in memory (sum of all msgs). Accounts for >>> + * only userspace overhead; ignoring any in-kernel rbtree nodes. >>> + */ >>> + unsigned long qsize; >>> >>> But no big deal in any case. >>> >>> I think this is the right approach, >> >> Me too. >> >>> but would still like to know if Doug >>> has any concerns about it. >> >> Looking on gmane, Doug does not appear to have been active on any >> lists since late May! Not sure why. > > I responded yesterday in the v2 patch thread I believe. In any case, I > think this patch is fine, and can go to stable. This patch doesn't > actually change the math related to the rlimit checks (which is the main > thing I wanted to correct in my original patches), instead it corrects a > mistake I made. At the time, I mistakenly thought that the qsize > included the current message data total + the struct msg_msg size total. > It didn't, it was just the current user data total. I added the rbtree > nodes in order to keep the total accurate but I shouldn't have added the > rbtree nodes to this total because none of the other kernel usage was > previously included. > > Acked-by: Doug Ledford <dledford@xxxxxxxxxx> + Acked-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx> Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
