Re: [RFC PATCH v2 1/5] mm: make HPAGE_PxD_{SHIFT,MASK,SIZE} always available

[Mike Rapoport <rppt@kernel.org>]

On Fri, Jul 10, 2020 at 05:57:46PM +0100, Matthew Wilcox wrote:
> On Fri, Jul 10, 2020 at 12:40:37PM -0400, Andrea Arcangeli wrote:
> > Hello Hugh and Mike,
> > 
> > On Mon, Jul 06, 2020 at 10:07:34PM -0700, Hugh Dickins wrote:
> > > Adding Andrea to Cc, he's the one who structured it that way,
> > > and should be consulted.
> > >
> > > I'm ambivalent myself.  Many's the time I've been irritated by the
> > > BUILD_BUG() in HPAGE_etc, and it's responsible for very many #ifdef
> > > CONFIG_TRANSPARENT_HUGEPAGEs or IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)s
> > > that you find uglily scattered around the source.
> > > 
> > > But that's the point of it: it's warning when you write code peculiar
> > > to THP, that is going to bloat the build of kernels without any THP.
> > > 
> > > So although I've often been tempted to do as you suggest, I've always
> > > ended up respecting Andrea's intention, and worked around it instead
> > > (sometimes with #ifdef or IS_ENABLED(), sometimes with
> > > PMD_{SHIFT,MASK_SIZE}, sometimes with a local definition).
> > 
> > The only other reasons that comes to mind in addition of optimizing
> > the bloat away at build time is to make it easier to identify the THP
> > code and to make it explicit that hugetlbfs shouldn't us it or it
> > could be wrong on some arches.
> > 
> > However for this case the BUILD_BUG() looks right and this doesn't
> > look like a false positive.
> > 
> > This patchset has nothing to do THP, so it'd be more correct to use
> > MAX_ORDER whenever the fragmentation is about the buddy (doesn't look
> > the case here) or PUD_SIZE/ORDER/PMD_SIZE/ORDER if the objective is
> > not to unnecessarily split extra and unrelated hugepud/hugepmds in the
> > direct mapping (as in this case).
> > 
> > The real issue exposed by the BUILD_BUG is the lack of PMD_ORDER
> > definition and fs/dax.c already run into and it solved it locally in the
> > dax.c file:
> > 
> > /* The order of a PMD entry */
> > #define PMD_ORDER	(PMD_SHIFT - PAGE_SHIFT)
> > 
> > The fact it's not just this patch but also dax.c that run into the
> > same issue, makes me think PMD_ORDER should be defined and then you
> > can use PMD_* and PUD_* for this non-THP purpose.
> 
> We'll run into some namespace issues.
> 
> arch/arm/kernel/head.S:#define PMD_ORDER        3
> arch/arm/kernel/head.S:#define PMD_ORDER        2
> arch/mips/include/asm/pgtable-32.h:#define PMD_ORDER    aieeee_attempt_to_allocate_pmd
> arch/mips/include/asm/pgtable-64.h:#define PMD_ORDER            0
> arch/parisc/include/asm/pgtable.h:#define PMD_ORDER     1 /* Number of pages per pmd */

This can be easily solved with, e.g.

#define PMD_PAGE_ORDER (PMD_SHIFT - PAGE_SHIFT)

or by renaming the current defines to PMD_ALLOC_ORDER.


> > Then the question if to remove the BUILD_BUG becomes orthogonal to
> > this patchset, but I don't see much value in retaining HPAGE_PMD/PUD_*
> > unless the BUILD_BUG is retained too, because this patchset already
> > hints that without the BUILD_BUG() the HPAGE_PMD_* definitions would
> > likely spill into non THP paths and they would lose also the only
> > value left (the ability to localize the THP code paths). So I wouldn't
> > be against removing the BUILD_BUG if it's causing maintenance
> > overhead, but then I would drop HPAGE_PMD_* too along with it or it
> > may just cause confusion.
> 
> btw, using the hpage_ prefix already caused one problem in the hugetlb
> code:
> 
> https://lore.kernel.org/linux-mm/20200629185003.97202-1-mike.kravetz@oracle.com/
> 
> I'd suggest we rename these to THP_PMD_* and THP_PUD_* to make it clear
> they're only for the THP case.

I agree that THP_PMD_* and THP_PUD_* would be less confusing if we are
to differentiate THP and non-THP usage of 2nd and 3rd level leaf pages.

-- 
Sincerely yours,
Mike.