> This patch establishes fields within the vm_area_struct type to store the > virtual page offset of VMAs. > > The virtual page offset of a VMA is equal to vma->vm_start >> PAGE_SHIFT if > they are unfaulted or were not remapped, otherwise it is equal to this > value at the point of first fault. > > Currently, anonymous folios belonging to CoW'd MAP_PRIVATE-mapped > file-backed VMAs are tracked by their file offset. By adding virtual offset > as a property of VMAs, we can now track them by their virtual page offset > instead. > > By tracking this, we provide the means by which to eliminate this > inconsistency, and more importantly lay the foundations for future work for > the scalable CoW anonymous rmap rework. > > This patch simply adds the fields and some simple helpers. Subsequent > patches will update mm code to make use of these fields correctly. > > The fields chosen are packed in the VMA such that, for 64-bit kernel > builds, no additional space is taken up. > > The first field is present on cacheline 0 containing key VMA fields, and > the second on cacheline 3, which contains file-backed reverse mapping > fields. > > Given the relative time spent accessing reverse mapping fields as well as > updating them, there shouldn't be any performance impact here from false > sharing. > > Update the VMA userland tests to account for this change. > > No callsites are updated yet, so no functional change intended. > > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]> > --- > include/linux/mm.h | 59 > +++++++++++++++++++++++++++++++++++++++++ > include/linux/mm_types.h | 12 +++++++++ > mm/vma.h | 14 ++++++++++ > mm/vma_init.c | 1 + > tools/testing/vma/include/dup.h | 26 ++++++++++++++++++ > 5 files changed, 112 insertions(+) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 87feaa5a2b78..59b98cc60402 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -4393,6 +4393,65 @@ static inline pgoff_t vma_last_pgoff(const struct > vm_area_struct *vma) > return vma_end_pgoff(vma) - 1; > } > > +/** > + * vma_start_virt_pgoff() - Get the virtual page offset of the start of @vma > + * @vma: The VMA whose virtual page offset is required. > + * > + * If unfaulted, then this is vma->vm_start >> PAGE_SHIFT, if faulted then > the > + * virtual page offset at the time of first fault. > + * > + * If the VMA is anonymous, this returns the same value as vma_start_pgoff(). > + * > + * This value is used for tracking MAP_PRIVATE file-backed mappings by their > + * virtual page offset. > + * > + * Returns: The virtual page offset of the start of @vma. > + */
Thanks. These looks much better to me. Reviewed-by: Xu Xin <[email protected]>
