madvise.2: add documentation for MADV_COLLAPSE

[thirdparty/man-pages.git] / man2 / madvise.2

diff --git a/man2/madvise.2 b/man2/madvise.2
index edf805740a8ea854a43e8c74f893d06927a07599..66aaf70e96ab7a7ee1d603f5c95732067a3f0114 100644 (file)
--- a/man2/madvise.2
+++ b/man2/madvise.2
@@ -386,9 +386,10 @@ set (see
 .BR prctl (2)).
 .IP
 The
-.B MADV_HUGEPAGE
+.BR MADV_HUGEPAGE ,
+.BR MADV_NOHUGEPAGE ,
 and
-.B MADV_NOHUGEPAGE
+.B MADV_COLLAPSE
 operations are available only if the kernel was configured with
 .B CONFIG_TRANSPARENT_HUGEPAGE
 and file/shmem memory is only supported if the kernel was configured with
@@ -401,6 +402,82 @@ and
 .I length
 will not be backed by transparent hugepages.
 .TP
+.BR MADV_COLLAPSE " (since Linux 6.1)"
+.\" commit 7d8faaf155454f8798ec56404faca29a82689c77
+.\" commit 34488399fa08faaf664743fa54b271eb6f9e1321
+Perform a best-effort synchronous collapse of
+the native pages mapped by the memory range
+into Transparent Huge Pages (THPs).
+.B MADV_COLLAPSE
+operates on the current state of memory of the calling process and
+makes no persistent changes or guarantees on how pages will be mapped,
+constructed,
+or faulted in the future.
+.IP
+.B MADV_COLLAPSE
+supports private anonymous pages (see
+.BR mmap (2)),
+shmem pages,
+and file-backed pages.
+See
+.B MADV_HUGEPAGE
+for general information on memory requirements for THP.
+If the range provided spans multiple VMAs,
+the semantics of the collapse over each VMA is independent from the others.
+If collapse of a given huge page-aligned/sized region fails,
+the operation may continue to attempt collapsing
+the remainder of the specified memory.
+.B MADV_COLLAPSE
+will automatically clamp the provided range to be hugepage-aligned.
+.IP
+All non-resident pages covered by the range
+will first be swapped/faulted-in,
+before being copied onto a freshly allocated hugepage.
+If the native pages compose the same PTE-mapped hugepage,
+and are suitably aligned,
+allocation of a new hugepage may be elided and
+collapse may happen in-place.
+Unmapped pages will have their data directly initialized to 0
+in the new hugepage.
+However,
+for every eligible hugepage-aligned/sized region to be collapsed,
+at least one page must currently be backed by physical memory.
+.IP
+.B MADV_COLLAPSE
+is independent of any sysfs
+(see
+.BR sysfs (5))
+setting under
+.IR /sys/kernel/mm/transparent_hugepage ,
+both in terms of determining THP eligibility,
+and allocation semantics.
+See Linux kernel source file
+.I Documentation/admin\-guide/mm/transhuge.rst
+for more information.
+.B MADV_COLLAPSE
+also ignores
+.B huge=
+tmpfs mount when operating on tmpfs files.
+Allocation for the new hugepage may enter direct reclaim and/or compaction,
+regardless of VMA flags
+(though
+.B VM_NOHUGEPAGE
+is still respected).
+.IP
+When the system has multiple NUMA nodes,
+the hugepage will be allocated from
+the node providing the most native pages.
+.IP
+If all hugepage-sized/aligned regions covered by the provided range were
+either successfully collapsed,
+or were already PMD-mapped THPs,
+this operation will be deemed successful.
+Note that this doesn't guarantee anything about
+other possible mappings of the memory.
+In the event multiple hugepage-aligned/sized areas fail to collapse,
+only the most-recently\[en]failed code will be set in
+.IR errno .
+.TP
 .BR MADV_DONTDUMP " (since Linux 3.4)"
 .\" commit 909af768e88867016f427264ae39d27a57b6a8ed
 .\" commit accb61fe7bb0f5c2a4102239e4981650f9048519
@@ -620,6 +697,11 @@ A kernel resource was temporarily unavailable.
 .B EBADF
 The map exists, but the area maps something that isn't a file.
 .TP
+.B EBUSY
+(for
+.BR MADV_COLLAPSE )
+Could not charge hugepage to cgroup: cgroup limit exceeded.
+.TP
 .B EFAULT
 .I advice
 is
@@ -717,6 +799,11 @@ maximum resident set size.
 Not enough memory: paging in failed.
 .TP
 .B ENOMEM
+(for
+.BR MADV_COLLAPSE )
+Not enough memory: could not allocate hugepage.
+.TP
+.B ENOMEM
 Addresses in the specified range are not currently
 mapped, or are outside the address space of the process.
 .TP