On 3/28/26 12:30 AM, Zi Yan wrote:
On 27 Mar 2026, at 12:22, Lance Yang wrote:
On Fri, Mar 27, 2026 at 11:00:26AM -0400, Zi Yan wrote:
On 27 Mar 2026, at 10:31, Lorenzo Stoakes (Oracle) wrote:
On Fri, Mar 27, 2026 at 10:26:53PM +0800, Baolin Wang wrote:
On 3/27/26 10:12 PM, Lorenzo Stoakes (Oracle) wrote:
On Fri, Mar 27, 2026 at 09:45:03PM +0800, Baolin Wang wrote:
On 3/27/26 8:02 PM, Lorenzo Stoakes (Oracle) wrote:
On Fri, Mar 27, 2026 at 05:44:49PM +0800, Baolin Wang wrote:
On 3/27/26 9:42 AM, Zi Yan wrote:
collapse_file() requires FSes supporting large folio with at least
PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. shmem with
huge option turned on also sets large folio order on mapping, so the check
also applies to shmem.
While at it, replace VM_BUG_ON with returning failure values.
Signed-off-by: Zi Yan <[email protected]>
---
mm/khugepaged.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d06d84219e1b..45b12ffb1550 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1899,8 +1899,11 @@ static enum scan_result collapse_file(struct mm_struct
*mm, unsigned long addr,
int nr_none = 0;
bool is_shmem = shmem_file(file);
- VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
- VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
+ /* "huge" shmem sets mapping folio order and passes the check below */
+ if (mapping_max_folio_order(mapping) < PMD_ORDER)
+ return SCAN_FAIL;
This is not true for anonymous shmem, since its large order allocation logic
is similar to anonymous memory. That means it will not call
mapping_set_large_folios() for anonymous shmem.
So I think the check should be:
if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
return SCAN_FAIL;
Hmm but in shmem_init() we have:
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY)
SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge;
else
shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */
/*
* Default to setting PMD-sized THP to inherit the global setting and
* disable all other multi-size THPs.
*/
if (!shmem_orders_configured)
huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
#endif
And shm_mnt->mnt_sb is the superblock used for anon shmem. Also
shmem_enabled_store() updates that if necessary.
So we're still fine right?
__shmem_file_setup() (used for anon shmem) calls shmem_get_inode() ->
__shmem_get_inode() which has:
if (sbinfo->huge)
mapping_set_large_folios(inode->i_mapping);
Shared for both anon shmem and tmpfs-style shmem.
So I think it's fine as-is.
I'm afraid not. Sorry, I should have been clearer.
First, anonymous shmem large order allocation is dynamically controlled via
the global interface (/sys/kernel/mm/transparent_hugepage/shmem_enabled) and
the mTHP interfaces
(/sys/kernel/mm/transparent_hugepage/hugepages-*kB/shmem_enabled).
This means that during anonymous shmem initialization, these interfaces
might be set to 'never'. so it will not call mapping_set_large_folios()
because sbinfo->huge is 'SHMEM_HUGE_NEVER'.
Even if shmem large order allocation is subsequently enabled via the
interfaces, __shmem_file_setup -> mapping_set_large_folios() is not called
again.
I see your point, oh this is all a bit of a mess...
It feels like entirely the wrong abstraction anyway, since at best you're
getting a global 'is enabled'.
I guess what happened before was we'd never call into this with ! r/o thp for fs
&& ! is_shmem.
Right.
But now we are allowing it, but should STILL be gating on !is_shmem so yeah your
suggestion is correct I think actualyl.
I do hate:
if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
As a bit of code though. It's horrible.
Indeed.
Let's abstract that...
It'd be nice if we could find a way to clean things up in the lead up to changes
in series like this instead of sticking with the mess, but I guess since it
mostly removes stuff that's ok for now.
I think this check can be removed from this patch.
During the khugepaged's scan, it will call thp_vma_allowable_order() to
check if the VMA is allowed to collapse into a PMD.
Specifically, within the call chain thp_vma_allowable_order() ->
__thp_vma_allowable_orders(), shmem is checked via
shmem_allowable_huge_orders(), while other FSes are checked via
file_thp_enabled().
But for madvise(MADV_COLLAPSE) case, IIRC, it ignores shmem huge config
and can perform collapse anyway. This means without !is_shmem the check
will break madvise(MADV_COLLAPSE). Let me know if I get it wrong, since
Right. That will break MADV_COLLAPSE, IIUC.
For MADV_COLLAPSE on anonymous shmem, eligibility is determined by the
TVA_FORCED_COLLAPSE path via shmem_allowable_huge_orders(), not by
whether the inode mapping got mapping_set_large_folios() at creation
time.
Using mmap(MAP_SHARED | MAP_ANONYMOUS):
- create time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=never
- collapse time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=always
With the !is_shmem guard, collapse succeeds. Without it, the same setup
fails with -EINVAL.
Right. So my suggestion is that the check should be:
if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
or just keep a single VM_WARN_ONCE() here, becuase I hope the
thp_vma_allowable_order() will filter out those FSes that do not
support large folios.
I was in that TVA_FORCED_COLLAPSE email thread but does not remember
everything there.
It sucks not to have an assert. Maybe in that case make it a
VM_WARN_ON_ONCE().
Will do that as I replied to David already.
I hate that you're left tracing things back like that...
For those other filesystems, Patch 5 has already added the following check,
which I think is sufficient to filter out those FSes that do not support
large folios:
if (mapping_max_folio_order(inode->i_mapping) < PMD_ORDER)
return false;
2 < 5, we won't tolerate bisection hazards.
Anonymous shmem behaves similarly to anonymous pages: it is controlled by
the 'shmem_enabled' interfaces and uses shmem_allowable_huge_orders() to
check for allowed large orders, rather than relying on
mapping_max_folio_order().
The mapping_max_folio_order() is intended to control large page allocation
only for tmpfs mounts. Therefore, I find the current code confusing and
think it needs to be fixed:
/* Don't consider 'deny' for emergencies and 'force' for testing */
if (sb != shm_mnt->mnt_sb && sbinfo->huge)
mapping_set_large_folios(inode->i_mapping);
Hi Baolin,
Do you want to send a fix for this?
Also I wonder how I can distinguish between anonymous shmem code and tmpfs code.
I thought they are the same thing except that they have different user
interface,
but it seems that I was wrong.
Sure. I can send a patch to make the code clear.
