On 11/20/25 20:32, David Hildenbrand (Red Hat) wrote: > On 11/20/25 10:25, Balbir Singh wrote: >> On 11/20/25 20:09, David Hildenbrand (Red Hat) wrote: >>> On 11/20/25 04:07, Balbir Singh wrote: >>>> Code refactoring of __folio_split() via helper >>>> __folio_freeze_and_split_unmapped() caused a regression with clang-20 >>>> with CONFIG_SHMEM=n, the compiler was not able to optimize away the >>>> call to shmem_uncharge() due to changes in nr_shmem_dropped. >>>> Fix this by checking for shmem_mapping() prior to calling >>>> shmem_uncharge(), shmem_mapping() returns false when CONFIG_SHMEM=n. >>>> >>>> smatch also complained about parameter end being used without >>>> initialization, which is a false positive, but keep the tool happy >>>> by sending in initialized parameters. end is initialized to 0. >>>> >>>> Add detailed documentation comments for folio_split_unmapped() >>>> >>>> Cc: Andrew Morton <[email protected]> >>>> Cc: David Hildenbrand <[email protected]> >>>> Cc: Zi Yan <[email protected]> >>>> Cc: Joshua Hahn <[email protected]> >>>> Cc: Rakie Kim <[email protected]> >>>> Cc: Byungchul Park <[email protected]> >>>> Cc: Gregory Price <[email protected]> >>>> Cc: Ying Huang <[email protected]> >>>> Cc: Alistair Popple <[email protected]> >>>> Cc: Oscar Salvador <[email protected]> >>>> Cc: Lorenzo Stoakes <[email protected]> >>>> Cc: Baolin Wang <[email protected]> >>>> Cc: "Liam R. Howlett" <[email protected]> >>>> Cc: Nico Pache <[email protected]> >>>> Cc: Ryan Roberts <[email protected]> >>>> Cc: Dev Jain <[email protected]> >>>> Cc: Barry Song <[email protected]> >>>> Cc: Lyude Paul <[email protected]> >>>> Cc: Danilo Krummrich <[email protected]> >>>> Cc: David Airlie <[email protected]> >>>> Cc: Simona Vetter <[email protected]> >>>> Cc: Ralph Campbell <[email protected]> >>>> Cc: Mika Penttilä <[email protected]> >>>> Cc: Matthew Brost <[email protected]> >>>> Cc: Francois Dugast <[email protected]> >>>> >>>> Signed-off-by: Balbir Singh <[email protected]> >>>> --- >>>> mm/huge_memory.c | 32 ++++++++++++++++++++++---------- >>>> 1 file changed, 22 insertions(+), 10 deletions(-) >>>> >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index 78a31a476ad3..c4267a0f74df 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -3751,6 +3751,7 @@ static int __folio_freeze_and_split_unmapped(struct >>>> folio *folio, unsigned int n >>>> int ret = 0; >>>> struct deferred_split *ds_queue; >>>> + VM_WARN_ON_ONCE(!mapping && end != 0); >>> >>> You could drop the "!= 0" >> >> Ack >> >> VM_WARN_ONE(!mapping && end); >> >>> >>>> /* Prevent deferred_split_scan() touching ->_refcount */ >>>> ds_queue = folio_split_queue_lock(folio); >>>> if (folio_ref_freeze(folio, 1 + extra_pins)) { >>>> @@ -3919,7 +3920,7 @@ static int __folio_split(struct folio *folio, >>>> unsigned int new_order, >>>> int nr_shmem_dropped = 0; >>>> int remap_flags = 0; >>>> int extra_pins, ret; >>>> - pgoff_t end; >>>> + pgoff_t end = 0; >>>> bool is_hzp; >>>> VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); >>>> @@ -4049,7 +4050,7 @@ static int __folio_split(struct folio *folio, >>>> unsigned int new_order, >>>> local_irq_enable(); >>>> - if (nr_shmem_dropped) >>>> + if (mapping && shmem_mapping(mapping) && nr_shmem_dropped) >>>> shmem_uncharge(mapping->host, nr_shmem_dropped); >>> >>> That looks questionable. We shouldn't add runtime check to handle buildtime >>> things. >>> >>> Likely what you want is instead >>> >>> if (IS_ENABLED(CONFIG_SHMEM) && nr_shmem_dropped) >>> shmem_uncharge() >>> >> >> shmem_mapping() returns false for CONFIG_SHMEM=n and shmem_mapping() checks >> that the mapping >> is indeed for shmem ops before uncharging. Happy to change it if you like, >> your version is more readable > Good point, but the questionable thing is that it looks like nr_shmem_dropped > could be set for non-shmem mappings, when it's really just a compiler thing. > > What about handling it through a proper stub so we can keep this calling code > simple? > > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h > index 5b368f9549d67..e38cb01031200 100644 > --- a/include/linux/shmem_fs.h > +++ b/include/linux/shmem_fs.h > @@ -136,11 +136,15 @@ static inline bool shmem_hpage_pmd_enabled(void) > > #ifdef CONFIG_SHMEM > extern unsigned long shmem_swap_usage(struct vm_area_struct *vma); > +extern void shmem_uncharge(struct inode *inode, long pages); > #else > static inline unsigned long shmem_swap_usage(struct vm_area_struct *vma) > { > return 0; > } > +static inline void shmem_uncharge(struct inode *inode, long pages) > +{ > +} > #endif > extern unsigned long shmem_partial_swap_usage(struct address_space *mapping, > pgoff_t start, pgoff_t end); > @@ -194,7 +198,6 @@ static inline pgoff_t shmem_fallocend(struct inode > *inode, pgoff_t eof) > } > > extern bool shmem_charge(struct inode *inode, long pages); > -extern void shmem_uncharge(struct inode *inode, long pages); > > #ifdef CONFIG_USERFAULTFD > #ifdef CONFIG_SHMEM > >
Agreed, I would like to let this patch proceed and then immediately follow up patch along the lines of CONFIG_SHMEM as separate independent patch (independent of this series). What do you think? Balbir changes.
