mm: get huge pages for unaligned allocations spanning at least one huge page

When page faults trigger pgalloc.MemoryFile allocation, the size of the
allocation never exceeds one huge page, and if it is exactly one huge page in
size then it will be hugepage-aligned. However, when pgalloc.MemoryFile
allocation is triggered by memory-mapped I/O (e.g. a read() to unfaulted
memory), mmap(MAP_POPULATE), mlock(), or MM.Pin() (due to driver activity), the
allocated range may be hugepage-unaligned but include one or more aligned huge
pages; in these cases, split the allocation into up to three parts for the
sub-hugepage subrange at the beginning, the sub-hugepage subrange at the end,
and the hugepage-aligned subrange in between, so that the hugepage-aligned
subrange can use a hugepage-backed allocation.

PiperOrigin-RevId: 812961653

Author: nixprime

pkg/sentry/mm/pma.go

@@ -282,7 +282,34 @@ func (mm *MemoryManager) getPMAsInternalLocked(ctx context.Context, vseg vmaIter
 					allocAR := optAR.Intersect(hugeMaskAR)
 					// Don't back stacks with huge pages due to low utilization
 					// and because they're often fragmented by copy-on-write.
-					huge := mm.mf.HugepagesEnabled() && allocAR.IsHugePageAligned() && !vma.growsDown && !vma.isStack
+					mayHuge := mm.mf.HugepagesEnabled() && !vma.growsDown && !vma.isStack
+					huge := false
+					if mayHuge {
+						if allocAR.IsHugePageAligned() {
+							huge = true
+						} else {
+							startHugeRoundUp, ok := allocAR.Start.HugeRoundUp()
+							endHugeRoundDown := allocAR.End.HugeRoundDown()
+							if ok && startHugeRoundUp < endHugeRoundDown {
+								// allocAR contains at least one full aligned
+								// huge page.
+								if allocAR.Start != startHugeRoundUp {
+									// Shorten allocAR to exclude full huge
+									// pages, so that a later iteration of this
+									// loop can allocate huge pages for the
+									// hugepage-aligned subrange.
+									allocAR.End = startHugeRoundUp
+								} else {
+									// Shorten allocAR to include only full
+									// huge pages, so that a later iteration of
+									// this loop can allocate small pages for
+									// the sub-hugepage tail of allocAR.
+									allocAR.End = endHugeRoundDown
+									huge = true
+								}
+							}
+						}
+					}
 					allocOpts := pgalloc.AllocOpts{
 						Kind:    usage.Anonymous,
 						MemCgID: memCgID,