On Thu, Nov 15, 2018 at 02:47:19PM -0800, Andrew Morton wrote:
On Thu, 15 Nov 2018 17:37:18 -0500 Sasha Levin <sashal@xxxxxxxxxx> wrote:On Thu, Nov 15, 2018 at 02:08:10PM -0800, Andrew Morton wrote: >On Tue, 13 Nov 2018 00:52:51 -0500 Sasha Levin <sashal@xxxxxxxxxx> wrote: > >> From: Jann Horn <jannh@xxxxxxxxxx> >> >> [ Upstream commit f0ecf25a093fc0589f0a6bc4c1ea068bbb67d220 ] >> >> Having two gigantic arrays that must manually be kept in sync, including >> ifdefs, isn't exactly robust. To make it easier to catch such issues in >> the future, add a BUILD_BUG_ON(). >> >> ... >> >> --- a/mm/vmstat.c >> +++ b/mm/vmstat.c >> @@ -1189,6 +1189,8 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos) >> stat_items_size += sizeof(struct vm_event_state); >> #endif >> >> + BUILD_BUG_ON(stat_items_size != >> + ARRAY_SIZE(vmstat_text) * sizeof(unsigned long)); >> v = kmalloc(stat_items_size, GFP_KERNEL); >> m->private = v; >> if (!v) > >I don't think there's any way in which this can make a -stable kernel >more stable! > > >Generally, I consider -stable in every patch I merge, so for each patch >which doesn't have cc:stable, that tag is missing for a reason. > >In other words, your criteria for -stable addition are different from >mine. > >And I think your criteria differ from those described in >Documentation/process/stable-kernel-rules.rst. > >So... what is your overall thinking on patch selection? Indeed, this doesn't fix anything. My concern is that in the future, we will pull a patch that will cause the issue described here, and that issue will only be relevant on stable. It is very hard to debug this, and I suspect that stable kernels will still pass all their tests with flying colors. As an example, consider the case where commit 28e2c4bb99aa ("mm/vmstat.c: fix outdated vmstat_text") is backported to a kernel that doesn't have commit 7a9cdebdcc17 ("mm: get rid of vmacache_flush_all() entirely"). I also felt safe with this patch since it adds a single BUILD_BUG_ON() which does nothing during runtime, so the chances it introduces anything beyond a build regression seemed to be slim to none.Well OK. But my question was general and covers basically every autosel patch which originated in -mm.
Sure. I picked 3 patches that show up on top when I google for AUTOSEL in linux-mm, maybe they'll be a good example to help me understand why they were not selected. This one fixes a case where too few struct pages are allocated when using mirrorred memory: https://marc.info/?l=linux-mm&m=154211933211147&w=2 Race condition with memory hotplug due to missing locks: https://marc.info/?l=linux-mm&m=154211934011188&w=2 Raising an OOM event that causes issues in userspace when no OOM has actually occured: https://marc.info/?l=linux-mm&m=154211939811582&w=2 I think that all 3 cases represent a "real" bug users can hit, and I honestly don't know why they were not tagged for stable. -- Thanks, Sasha
