Re: [PATCH v1] madvise.2: Document MADV_POPULATE_READ and MADV_POPULATE_WRITE

MADV_POPULATE_READ and MADV_POPULATE_WRITE have been merged into
upstream Linux via commit 4ca9b3859dac ("mm/madvise: introduce
MADV_POPULATE_(READ|WRITE) to prefault page tables"), part of v5.14-rc1.

Let's document the behavior and error conditions of these new madvise()
options.

Cc: Alejandro Colomar <redacted>
Cc: Michael Kerrisk <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Jann Horn <jannh@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Linux API <redacted>
Cc: linux-mm@kvack.org
Signed-off-by: David Hildenbrand <redacted>
---
 man2/madvise.2 | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)

diff --git a/man2/madvise.2 b/man2/madvise.2
index f1f384c0c..3ec8c53a7 100644
--- a/man2/madvise.2
+++ b/man2/madvise.2

@@ -469,6 +469,59 @@ If a page is file-backed and dirty, it will be written back to the backing
 storage.
 The advice might be ignored for some pages in the range when it is not
 applicable.
+.TP
+.BR MADV_POPULATE_READ " (since Linux 5.14)
+Populate (prefault) page tables readable for the whole range without actually
+reading. Depending on the underlying mapping, map the shared zeropage,
+preallocate memory or read the underlying file; files with holes might or
+might not preallocate blocks.
+Do not generate
+.B SIGBUS
+when populating fails, return an error instead.
+.IP
+If
+.B MADV_POPULATE_READ
+succeeds, all page tables have been populated (prefaulted) readable once.
+If
+.B MADV_POPULATE_READ
+fails, some page tables might have been populated.
+.IP
+.B MADV_POPULATE_READ
+cannot be applied to mappings without read permissions
+and special mappings marked with the kernel-internal
+.B VM_PFNMAP
+and
+.BR VM_IO .
+.IP
+Note that with
+.BR MADV_POPULATE_READ ,
+the process can be killed at any moment when the system runs out of memory.
+.TP
+.BR MADV_POPULATE_WRITE " (since Linux 5.14)
+Populate (prefault) page tables writable for the whole range without actually
+writing. Depending on the underlying mapping, preallocate memory or read the
+underlying file; files with holes will preallocate blocks.
+Do not generate
+.B SIGBUS
+when populating fails, return an error instead.
+.IP
+If
+.B MADV_POPULATE_WRITE
+succeeds, all page tables have been populated (prefaulted) writable once.
+If
+.B MADV_POPULATE_WRITE
+fails, some page tables might have been populated.
+.IP
+.B MADV_POPULATE_WRITE
+cannot be applied to mappings without write permissions
+and special mappings marked with the kernel-internal
+.B VM_PFNMAP
+and
+.BR VM_IO .
+.IP
+Note that
+.BR MADV_POPULATE_WRITE ,
+the process can be killed at any moment when the system runs out of memory.
 .SH RETURN VALUE
 On success,
 .BR madvise ()

@@ -533,6 +586,17 @@ or
 .BR VM_PFNMAP
 ranges.
 .TP
+.B EINVAL
+.I advice
+is
+.B MADV_POPULATE_READ
+or
+.BR MADV_POPULATE_WRITE ,
+but the specified address range includes ranges with insufficient permissions,
+.B VM_IO
+or
+.BR VM_PFNMAP.
+.TP
 .B EIO
 (for
 .BR MADV_WILLNEED )

@@ -548,6 +612,14 @@ Not enough memory: paging in failed.
 Addresses in the specified range are not currently
 mapped, or are outside the address space of the process.
 .TP
+.B ENOMEM
+.I advice
+is
+.B MADV_POPULATE_READ
+or
+.BR MADV_POPULATE_WRITE ,
+but populating (prefaulting) page tables failed.
+.TP
 .B EPERM
 .I advice
 is

@@ -555,6 +627,14 @@ is
 but the caller does not have the
 .B CAP_SYS_ADMIN
 capability.
+.TP
+.B EHWPOISON
+.I advice
+is
+.B MADV_POPULATE_READ
+or
+.BR MADV_POPULATE_WRITE ,
+and a HW poisoned page is encountered.
 .SH VERSIONS
 Since Linux 3.18,
 .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb