Messages in this thread | | | Date | Mon, 12 Sep 2016 21:10:37 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps |
| |
On Mon 12-09-16 08:01:06, Dave Hansen wrote: > On 09/12/2016 05:54 AM, Michal Hocko wrote: > >> > In order to fix this bug, we make 'file->version' indicate the end address > >> > of current VMA > > Doesn't this open doors to another weird cases. Say B would be partially > > unmapped (tail of the VMA would get unmapped and reused for a new VMA. > > In the end, this interface isn't about VMAs. It's about addresses, and > we need to make sure that the _addresses_ coming out of it are sane. In > the case that a VMA was partially unmapped, it doesn't make sense to > show the "new" VMA because we already had some output covering the > address of the "new" VMA from the old one.
OK, that is a fair point and it speaks for caching the vm_end rather than vm_start+skip.
> > I am not sure we provide any guarantee when there are more read > > syscalls. Hmm, even with a single read() we can get inconsistent results > > from different threads without any user space synchronization. > > Yeah, very true. But, I think we _can_ at least provide the following > guarantees (among others): > 1. addresses don't go backwards > 2. If there is something at a given vaddr during the entirety of the > life of the smaps walk, we will produce some output for it.
I guess we also want 3. no overlaps with previously printed values (assuming two subsequent reads without seek).
the patch tries to achieve the last part as well AFAICS but I guess this is incomplete because at least /proc/<pid>/smaps will report counters for the full vma range while the header (aka show_map_vma) will report shorter (non-overlapping) range. I haven't checked other files which use m_{start,next}
Considering how this all can be tricky and how partial reads can be confusing and even misleading I am really wondering whether we should simply document that only full reads will provide a sensible results.
-- Michal Hocko SUSE Labs
| |