Re: [ewg] [PATCH v2] libibverbs: ibv_fork_init() and libhugetlbfs

[Roland Dreier <rdreier-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

 > We thought about this too, but in some special cases, we do not know the
 > correct page size of a memory range. For example when getting a 16M chunk
 > from a 16M huge page region which is also aligned to 16M, the first madvise()
 > will work fine and the code will assume that the page size is 64K.

I see ... yes, that does break my idea completely.

OK, another half-baked idea: what if we pay attention to when
madvise(DOFORK) fails as well as well madvise(DONTFORK) fails, and use
that as a hit that we better check the page size?

Perhaps this adds too much complexity ... in which case your idea:

 > As this issue was not present in version 2 of the code, but there we had
 > a big performance penalty, I suggest the following: we could go back to
 > version 2 and introduce a new RDMAV_HUGEPAGE_SAFE env variable to let the user
 > decide between huge page support and better performance (the same approach we
 > use for the COW protection itself).

seems like a reasonable alternative.

Thanks,
  Roland
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Roland Dreier <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org> || For corporate legal information go to:
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