Re: [PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Lorenzo Stoakes <ljs@kernel.org>]

On Tue, Apr 07, 2026 at 02:42:59PM -0700, Andrew Morton wrote:
> On Tue, 7 Apr 2026 09:38:19 +0100 "Lorenzo Stoakes (Oracle)" <ljs@kernel.org> wrote:
>
> > On Wed, Apr 01, 2026 at 01:31:45PM -0700, Andrew Morton wrote:
> > > On Wed, 1 Apr 2026 09:14:35 +0100 "Lorenzo Stoakes (Oracle)" <ljs@kernel.org> wrote:
> > >
> > > > From: "Lorenzo Stoakes (Oracle)" <ljs@kernel.org>
> > > > Date: Tue, 31 Mar 2026 13:11:18 +0100
> > > > Subject: [PATCH] mm/khugepaged: fix issue with tracking lock
> > > >
> > > > We are incorrectly treating lock_dropped to track both whether the lock is
> > > > currently held and whether or not the lock was ever dropped.
> > > >
> > > > Update this change to account for this.
> > > >
> > > > Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> > >
> > > Not sure if I should add this?
> > >
> > > Should it have
> > >
> > > Fixes: 330f3758a3bc ("mm/khugepaged: unify khugepaged and madv_collapse
> > > with collapse_single_pmd()")?
> >
> > (Sorry only just noticed this)
> >
> > I mean we do need this patch, if you plan to do this as a fix follow up then
> > should have a fixed tag, but I do worry about bisection hazard by leaving this
> > as-is.
> >
> > We definitely either need this patch folded in or as a fix that gets sent
> > alongside the change.
>
> I added this to mm-stable a couple of days ago as 5a62019807da
> ("mm/khugepaged: fix issue with tracking lock"), below.  It does have
> the Fixes: so I think we're good.

Ack LGTM thanks!

Sorry was going through old mail after I realised I didn't know how to use
neomutt properly and missed a bunch :)

Cheers, Lorenzo

>
>
> commit 5a62019807da4a7add0717c962ac83b23dd12b2c
> Author: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> Date:   Tue Mar 31 13:11:18 2026 +0100
>
>     mm/khugepaged: fix issue with tracking lock
>
>     We are incorrectly treating lock_dropped to track both whether the lock is
>     currently held and whether or not the lock was ever dropped.
>
>     Update this change to account for this.
>
>     Link: https://lkml.kernel.org/r/7760c811-e100-4d40-9217-0813c28314be@lucifer.local
>     Fixes: 330f3758a3bc ("mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()")
>     Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
>     Reviewed-by: Lance Yang <lance.yang@linux.dev>
>     Reviewed-by: Nico Pache <npache@redhat.com>
>     Cc: Alistair Popple <apopple@nvidia.com>
>     Cc: Andrea Arcangeli <aarcange@redhat.com>
>     Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>     Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>     Cc: Barry Song <baohua@kernel.org>
>     Cc: Brendan Jackman <jackmanb@google.com>
>     Cc: Byungchul Park <byungchul@sk.com>
>     Cc: Catalin Marinas <catalin.marinas@arm.com>
>     Cc: David Hildenbrand <david@kernel.org>
>     Cc: David Rientjes <rientjes@google.com>
>     Cc: Dev Jain <dev.jain@arm.com>
>     Cc: Gregory Price <gourry@gourry.net>
>     Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
>     Cc: Hugh Dickins <hughd@google.com>
>     Cc: Jan Kara <jack@suse.cz>
>     Cc: Jann Horn <jannh@google.com>
>     Cc: Johannes Weiner <hannes@cmpxchg.org>
>     Cc: Jonathan Corbet <corbet@lwn.net>
>     Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>     Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>     Cc: Liam Howlett <liam.howlett@oracle.com>
>     Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
>     Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>     Cc: Matthew Brost <matthew.brost@intel.com>
>     Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
>     Cc: Michal Hocko <mhocko@suse.com>
>     Cc: Mike Rapoport <rppt@kernel.org>
>     Cc: Nanyong Sun <sunnanyong@huawei.com>
>     Cc: Pedro Falcato <pfalcato@suse.de>
>     Cc: Peter Xu <peterx@redhat.com>
>     Cc: Rafael Aquini <raquini@redhat.com>
>     Cc: Rakie Kim <rakie.kim@sk.com>
>     Cc: Randy Dunlap <rdunlap@infradead.org>
>     Cc: Ryan Roberts <ryan.roberts@arm.com>
>     Cc: Shivank Garg <shivankg@amd.com>
>     Cc: Steven Rostedt <rostedt@goodmis.org>
>     Cc: Suren Baghdasaryan <surenb@google.com>
>     Cc: Takashi Iwai (SUSE) <tiwai@suse.de>
>     Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>     Cc: Usama Arif <usamaarif642@gmail.com>
>     Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
>     Cc: Vlastimil Babka <vbabka@suse.cz>
>     Cc: Wei Yang <richard.weiyang@gmail.com>
>     Cc: Will Deacon <will@kernel.org>
>     Cc: Yang Shi <yang@os.amperecomputing.com>
>     Cc: Zach O'Keefe <zokeefe@google.com>
>     Cc: Zi Yan <ziy@nvidia.com>
>     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d21348b85a59..b8452dbdb043 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2828,6 +2828,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>  	unsigned long hstart, hend, addr;
>  	enum scan_result last_fail = SCAN_FAIL;
>  	int thps = 0;
> +	bool mmap_unlocked = false;
>
>  	BUG_ON(vma->vm_start > start);
>  	BUG_ON(vma->vm_end < end);
> @@ -2850,10 +2851,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>  	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
>  		enum scan_result result = SCAN_FAIL;
>
> -		if (*lock_dropped) {
> +		if (mmap_unlocked) {
>  			cond_resched();
>  			mmap_read_lock(mm);
> -			*lock_dropped = false;
> +			mmap_unlocked = false;
> +			*lock_dropped = true;
>  			result = hugepage_vma_revalidate(mm, addr, false, &vma,
>  							 cc);
>  			if (result  != SCAN_SUCCEED) {
> @@ -2864,7 +2866,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>  			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
>  		}
>
> -		result = collapse_single_pmd(addr, vma, lock_dropped, cc);
> +		result = collapse_single_pmd(addr, vma, &mmap_unlocked, cc);
>
>  		switch (result) {
>  		case SCAN_SUCCEED:
> @@ -2893,8 +2895,10 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>
>  out_maybelock:
>  	/* Caller expects us to hold mmap_lock on return */
> -	if (*lock_dropped)
> +	if (mmap_unlocked) {
> +		*lock_dropped = true;
>  		mmap_read_lock(mm);
> +	}
>  out_nolock:
>  	mmap_assert_locked(mm);
>  	mmdrop(mm);
>