| Message ID | 20211208133930.599253-1-lixinhai.lxh@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/gup.c: stricter check on THP migration entry during follow_pmd_mask | expand |
On Wed, 8 Dec 2021 21:39:30 +0800 Li Xinhai <lixinhai.lxh@gmail.com> wrote: > When BUG_ON check for THP migration entry, we need also consider > !thp_migration_supported() case, becaouse it is invalid at the code > context. Now, make this check been covered without adding extra cost. > > Because pmdval instead of *pmd is read, so just call > pmd_migration_entry_wait(). > > Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com> > --- > mm/gup.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 2c51e9748a6a..49f490bae030 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > if (!pmd_present(pmdval)) { > if (likely(!(flags & FOLL_MIGRATION))) > return no_page_table(vma, flags); > - VM_BUG_ON(thp_migration_supported() && > + VM_BUG_ON(!thp_migration_supported() || I don't get this. thp_migration_supported() evaluates to a compile-time constant. What is the point in doing a runtime BUG based on a thing which is known at compile-time? > !is_pmd_migration_entry(pmdval)); > - if (is_pmd_migration_entry(pmdval)) > - pmd_migration_entry_wait(mm, pmd); > + pmd_migration_entry_wait(mm, pmd); > pmdval = READ_ONCE(*pmd); > /* > * MADV_DONTNEED may convert the pmd to null because
On 12/9/21 7:35 AM, Andrew Morton wrote: > On Wed, 8 Dec 2021 21:39:30 +0800 Li Xinhai <lixinhai.lxh@gmail.com> wrote: > >> When BUG_ON check for THP migration entry, we need also consider >> !thp_migration_supported() case, becaouse it is invalid at the code >> context. Now, make this check been covered without adding extra cost. >> >> Because pmdval instead of *pmd is read, so just call >> pmd_migration_entry_wait(). >> >> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com> >> --- >> mm/gup.c | 5 ++--- >> 1 file changed, 2 insertions(+), 3 deletions(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index 2c51e9748a6a..49f490bae030 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, >> if (!pmd_present(pmdval)) { >> if (likely(!(flags & FOLL_MIGRATION))) >> return no_page_table(vma, flags); >> - VM_BUG_ON(thp_migration_supported() && >> + VM_BUG_ON(!thp_migration_supported() || > > I don't get this. thp_migration_supported() evaluates to a > compile-time constant. What is the point in doing a runtime BUG based > on a thing which is known at compile-time? > Yes, support thp migration is compile-time known. The exsiting VM_BUG_ON will be nothing if thp_migration_supported() equal to false, so pmdval other than migration entry will not be caught. Now, with the patch, if thp_migration_supported() equal to false, we will have VM_BUG_ON(1) so to catch above error case. >> !is_pmd_migration_entry(pmdval)); >> - if (is_pmd_migration_entry(pmdval)) >> - pmd_migration_entry_wait(mm, pmd); >> + pmd_migration_entry_wait(mm, pmd); >> pmdval = READ_ONCE(*pmd); >> /* >> * MADV_DONTNEED may convert the pmd to null because >
diff --git a/mm/gup.c b/mm/gup.c index 2c51e9748a6a..49f490bae030 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, if (!pmd_present(pmdval)) { if (likely(!(flags & FOLL_MIGRATION))) return no_page_table(vma, flags); - VM_BUG_ON(thp_migration_supported() && + VM_BUG_ON(!thp_migration_supported() || !is_pmd_migration_entry(pmdval)); - if (is_pmd_migration_entry(pmdval)) - pmd_migration_entry_wait(mm, pmd); + pmd_migration_entry_wait(mm, pmd); pmdval = READ_ONCE(*pmd); /* * MADV_DONTNEED may convert the pmd to null because
When BUG_ON check for THP migration entry, we need also consider !thp_migration_supported() case, becaouse it is invalid at the code context. Now, make this check been covered without adding extra cost. Because pmdval instead of *pmd is read, so just call pmd_migration_entry_wait(). Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com> --- mm/gup.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)