On Thu, Nov 23 2017, Ian Kent wrote: > > Hey Neil, I'm looking at this again because RH QE have complained about > a regression test failing with a kernel that has this change. > > Maybe I'm just dumb but I though a "find <base directory> <options>" > would, well, just look at the contents below <base directory> but an > strace shows that it reads and calls fstatat() on "every entry in the > mount table" regardless of the path. weird ... I can only get find to look at the mount table if given the -fstyp option, and even then it doesn't fstatat anything that isn't in the tree it is searching. > > And with the move of userspace to use /proc based mount tables (one > example being the symlink of /etc/mtab into /proc) even modest sized > direct mount maps will be a problem with every entry getting mounted. But the patch in question is only about indirect mount maps, isn't it? How is it relevant to direct mount maps? > > Systems will cope with this fine but larger systems not so much. > > If find does this then the user space changes needed to accommodate > this sort of change are almost certainly far more than I expected. > > I think this is an example of the larger problem I'm faced with and > this change was was meant to be a starting point for resolution. > > The most obvious symptom of the problem is auto-mounts no longer able > to be expired due to being re-mounted immediately after expire. Another > symptom is unwanted (by the user) accesses causing unexpected auto-mount > attempts. > > I believe this monitoring of the mount table is what leads to excessive > CPU consumption I've seen, usually around six processes, under heavy > mount activity. And following this, when the mount table is large and > there is "no mount activity" two of the six processes continue to consume > excessive CPU, until the mount table shrinks. > > So now I'm coming around to the idea of reverting this change ..... and > going back to the drawing board. I can well imaging that a large mount table could cause problems for applications that are written to expect one, and I can imagine that autofs could cause extra issues for such a program as it might change the mount table more often. But I haven't yet worked out how this is related to the patch in question.... Thanks, NeilBrown
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