On 22.07.24 17:31, Peter Xu wrote:
On Mon, Jul 22, 2024 at 03:29:43PM +0200, David Hildenbrand wrote:
On 18.07.24 00:02, Peter Xu wrote:
This is an RFC series, so not yet for merging. Please don't be scared by
the code changes: most of them are code movements only.
This series is based on the dax mprotect fix series here (while that one is
based on mm-unstable):
[PATCH v3 0/8] mm/mprotect: Fix dax puds
https://lore.kernel.org/r/20240715192142.3241557-1-peterx@xxxxxxxxxx
Overview
========
This series doesn't provide any feature change. The only goal of this
series is to start decoupling two ideas: "THP" and "huge mapping". We
already started with having PGTABLE_HAS_HUGE_LEAVES config option, and this
one extends that idea into the code.
The issue is that we have so many functions that only compile with
CONFIG_THP=on, even though they're about huge mappings, and huge mapping is
a pretty common concept, which can apply to many things besides THPs
nowadays. The major THP file is mm/huge_memory.c as of now.
The first example of such huge mapping users will be hugetlb. We lived
until now with no problem simply because Linux almost duplicated all the
logics there in the "THP" files into hugetlb APIs. If we want to get rid
of hugetlb specific APIs and paths, this _might_ be the first thing we want
to do, because we want to be able to e.g., zapping a hugetlb pmd entry even
if !CONFIG_THP.
Then consider other things like dax / pfnmaps. Dax can depend on THP, then
it'll naturally be able to use pmd/pud helpers, that's okay. However is it
a must? Do we also want to have every new pmd/pud mappings in the future
to depend on THP (like PFNMAP)? My answer is no, but I'm open to opinions.
If anyone agrees with me that "huge mapping" (aka, PMD/PUD mappings that
are larger than PAGE_SIZE) is a more generic concept than THP, then I think
at some point we need to move the generic code out of THP code into a
common code base.
This is what this series does as a start.
Hi Peter!
From a quick glimpse, patch #1-#4 do make sense independent of patch #5.
I am not so sure about all of the code movement in patch #5. If large folios
are the future, then likely huge_memory.c should simply be the home for all
that logic.
Maybe the goal should better be to compile huge_memory.c not only for THP,
but also for other use cases that require that logic, and fence off all THP
specific stuff using #ifdef?
Not sure, though. But a lot of this code movements/churn might be avoidable.
I'm fine using ifdefs in the current fine, but IMHO it's a matter of
whether we want to keep huge_memory.c growing into even larger file, and
keep all large folio logics only in that file. Currently it's ~4000 LOCs.
Depends on "how much" for sure. huge_memory.c is currently on place 12
of the biggest files in mm/. So there might not be immediate cause for
action ... just yet :) [guess which file is on #2 :) ]
Nornally I don't see this as much of a "code churn" category, because it
doesn't changes the code itself but only move things. I personally also
prefer without code churns, but only in the case where there'll be tiny
little functional changes here and there without real benefit.
It's pretty unavoidable to me when one file grows too large and we'll need
to split, and in this case git doesn't have a good way to track such
movement..
Yes, that's what I mean.
I've been recently thinking if we should pursue a different direction:
Just as we recently relocated most follow_huge_* stuff into gup.c,
likely we should rather look into moving copy_huge_pmd, change_huge_pmd,
copy_huge_pmd ... into the files where they logically belong to.
In madvise.c, we've been doing that in some places already: For
madvise_cold_or_pageout_pte_range() we inline the code, but not for
madvise_free_huge_pmd().
pmd_trans_huge() would already compile to a NOP without
CONFIG_TRANSPARENT_HUGEPAGE, but to make that code avoid most
CONFIG_TRANSPARENT_HUGEPAGE, we'd need a couple more function stubs to
make the compiler happy while still being able to compile that code out
when not required.
The idea would be that e.g., pmd_leaf() would return "false" at compile
time if no active configuration (THP, HUGETLB, ...) would be active. So
we could just use pmd_leaf() similar to pmd_trans_huge() in relevant
code and have the compiler optimize it all out without putting it into
separate files.
That means, large folios and PMD/PUD mappings will become "more common"
and better integrated, without the need to jump between files.
Just some thought about an alternative that would make sense to me.
Irrelevant of this, just to mention I think there's still one option that I
at least can make the huge pfnmap depends on THP again which shouldn't be a
huge deal (I don't have any use case that needs huge pfnmap but disable
THP, anyway..), so this series isn't an immediate concern to me for that
route. But for a hugetlb rework this might be something we need to do,
because we simplly can't make CONFIG_HUGETLB rely on CONFIG_THP..
Yes, likely. FSDAX went a similar direction and called that FSDAX thing
a "THP" whereby it really doesn't have anything in common with a THP,
besides being partially mappable -- IMHO.
--
Cheers,
David / dhildenb
