On Sat, Oct 05, 2024 at 03:24:39PM +0200, Bert Karwatzki wrote: > Am Samstag, dem 05.10.2024 um 12:17 +0100 schrieb Lorenzo Stoakes: > > On Sat, Oct 05, 2024 at 07:41:14AM +0100, Lorenzo Stoakes wrote: > > > Writing a data range into a maple tree may involve overwriting a number of > > > existing entries that span across more than one node. Doing so invokes a > > > 'spanning' store. > > > > > > > [snip] > > > > Andrew - just to note that I have intentionally left stable off this, in > > order that wre can allow this to stabilise in the 6.12 release candidates. > > > > Up until 6.12 this bug seemed much harder to hit, and as far as I'm aware > > we've never had a bug report for it prior to this. > > I still suspect that this could have been the same error: > https://lkml.org/lkml/2024/8/28/1558 > When compiling the kernel without CONFIG_DEBUG_VM maple tree bug results in an > unkillable task, and when trying to kill it first produced the rwsem warning > (and soon after took down the whole system). > But I couldn't reproduce it with the given reproducer, either. > > Bert Karwatzki > Thanks for reminding me of that one! Yeah unfortunately that thread was very unproductive in that the reporter gave no feedback or further information. They spammed the list with a bunch of such reports many looking suspect... So it is possible, and I suspect that this bug may have caused some other 'weird' crashes that were non-repro in the past. The difference here may be that we (or rather specifically - you! :) finally found a way to reliable repro this to the point where we could diagnose it. As far as I can tell this could happen even with vma_iter_clear_gfp(), so old unmap/MAP_FIXED behaviour could have hit it. BUT a difference now is that we essentially combine the MAP_FIXED with a merge and overwrite everything in between, so this addition of up to 2 extra entries probably pushed it over the edge to make this event statistically likely enough for you to have hit it. Note in your case it took unmapping 6 (!) entries and merging another 2 for a total of an overwrite spanning 8 entries. Anyway, assuming we so no issues stabilising this in the rc's I will ping stable to get this backported and fix this everywhere.
