Re: [RFC PATCH 02/12] khugepaged: Generalize alloc_charge_folio()

[Dev Jain]

On 17/12/24 12:23 pm, Ryan Roberts wrote:
> On 16/12/2024 16:50, Dev Jain wrote:
> > Pass order to alloc_charge_folio() and update mTHP statistics.
> >
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> >   include/linux/huge_mm.h |  2 ++
> >   mm/huge_memory.c        |  4 ++++
> >   mm/khugepaged.c         | 13 +++++++++----
> >   3 files changed, 15 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 93e509b6c00e..8b6d0fed99b3 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -119,6 +119,8 @@ enum mthp_stat_item {
> >   	MTHP_STAT_ANON_FAULT_ALLOC,
> >   	MTHP_STAT_ANON_FAULT_FALLBACK,
> >   	MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
> > +	MTHP_STAT_ANON_COLLAPSE_ALLOC,
> > +	MTHP_STAT_ANON_COLLAPSE_ALLOC_FAILED,
> >   	MTHP_STAT_ZSWPOUT,
> >   	MTHP_STAT_SWPIN,
> >   	MTHP_STAT_SWPIN_FALLBACK,
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 2da5520bfe24..2e582fad4c77 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -615,6 +615,8 @@ static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
> >   DEFINE_MTHP_STAT_ATTR(anon_fault_alloc, MTHP_STAT_ANON_FAULT_ALLOC);
> >   DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
> >   DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
> > +DEFINE_MTHP_STAT_ATTR(anon_collapse_alloc, MTHP_STAT_ANON_COLLAPSE_ALLOC);
> > +DEFINE_MTHP_STAT_ATTR(anon_collapse_alloc_failed, MTHP_STAT_ANON_COLLAPSE_ALLOC_FAILED);
> >   DEFINE_MTHP_STAT_ATTR(zswpout, MTHP_STAT_ZSWPOUT);
> >   DEFINE_MTHP_STAT_ATTR(swpin, MTHP_STAT_SWPIN);
> >   DEFINE_MTHP_STAT_ATTR(swpin_fallback, MTHP_STAT_SWPIN_FALLBACK);
> > @@ -636,6 +638,8 @@ static struct attribute *anon_stats_attrs[] = {
> >   	&anon_fault_alloc_attr.attr,
> >   	&anon_fault_fallback_attr.attr,
> >   	&anon_fault_fallback_charge_attr.attr,
> > +	&anon_collapse_alloc_attr.attr,
> > +	&anon_collapse_alloc_failed_attr.attr,
> >   #ifndef CONFIG_SHMEM
> >   	&zswpout_attr.attr,
> >   	&swpin_attr.attr,
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 95643e6e5f31..02cd424b8e48 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1073,21 +1073,26 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> >   }
> >   
> >   static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
> > -			      struct collapse_control *cc)
> > +			      int order, struct collapse_control *cc)
> >   {
> >   	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
> >   		     GFP_TRANSHUGE);
> >   	int node = hpage_collapse_find_target_node(cc);
> >   	struct folio *folio;
> >   
> > -	folio = __folio_alloc(gfp, HPAGE_PMD_ORDER, node, &cc->alloc_nmask);
> > +	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask);
> >   	if (!folio) {
> >   		*foliop = NULL;
> >   		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
> > +		if (order != HPAGE_PMD_ORDER)
> > +			count_mthp_stat(order, MTHP_STAT_ANON_COLLAPSE_ALLOC_FAILED);
> Bug? We should be calling count_mthp_stat() for all orders, but only calling
> count_vm_event(THP_*) for PMD_ORDER, as per pattern laid out by other mTHP stats.

Ah okay.

> The aim is for existing THP stats (which are implicitly only counting PMD-sized
> THP) to continue only to count PMD-sized THP. It's a userspace ABI and we were
> scared of the potential to break things if we changed the existing counters'
> semantics.
>
> >   		return SCAN_ALLOC_HUGE_PAGE_FAIL;
> >   	}
> >   
> >   	count_vm_event(THP_COLLAPSE_ALLOC);
> > +	if (order != HPAGE_PMD_ORDER)
> > +		count_mthp_stat(order, MTHP_STAT_ANON_COLLAPSE_ALLOC);
> Same problem.
>
> Also, I agree with Baolin that we don't want "anon" in the title. This is a
> generic path used for file-backed memory. So once you fix the bug, the new stats
> will also be counting the file-backed memory too (although for now, only for
> PMD_ORDER).

Sure.

> > +
> >   	if (unlikely(mem_cgroup_charge(folio, mm, gfp))) {
> >   		folio_put(folio);
> >   		*foliop = NULL;
> > @@ -1124,7 +1129,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >   	 */
> >   	mmap_read_unlock(mm);
> >   
> > -	result = alloc_charge_folio(&folio, mm, cc);
> > +	result = alloc_charge_folio(&folio, mm, order, cc);
> Where is order coming from? I'm guessing that's added later, so this patch won't
> compile on it's own? Perhaps HPAGE_PMD_ORDER for now?

Okay yes, this won't compile on its own. I'll ensure sequential buildability next time.

> >   	if (result != SCAN_SUCCEED)
> >   		goto out_nolock;
> >   
> > @@ -1850,7 +1855,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >   	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
> >   	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
> >   
> > -	result = alloc_charge_folio(&new_folio, mm, cc);
> > +	result = alloc_charge_folio(&new_folio, mm, HPAGE_PMD_ORDER, cc);
> >   	if (result != SCAN_SUCCEED)
> >   		goto out;
> >