Re: [PATCH man-pages v3 4/4] madvise.2: add documentation for MADV_COLLAPSE

["Zach O'Keefe" <zokeefe@google.com>]

On Sun, Dec 11, 2022 at 9:59 AM Alejandro Colomar
<alx.manpages@gmail.com> wrote:
>
> Hi Zach,

Hey Alex,

> On 10/22/22 00:33, Zach OKeefe wrote:
> > From: Zach O'Keefe <zokeefe@google.com>
> >
> > Linux 6.1 introduced MADV_COLLAPSE in upstream commit 7d8faaf15545
> > ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse") and
> > upstream commit 34488399fa08 ("mm/madvise: add file and shmem support to
> > MADV_COLLAPSE").  Update the man-pages for madvise(2) and
> > process_madvise(2).
> >
> > Link: https://lore.kernel.org/linux-mm/20220922224046.1143204-1-zokeefe@google.com/
> > Link: https://lore.kernel.org/linux-mm/20220706235936.2197195-1-zokeefe@google.com/
> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
>
> Please see a few comments below.
>

Thanks for the mail. So, this patch was taken as commit b106cd5bf
("madvise.2: add documentation for MADV_COLLAPSE"). Some of your
comments below were
applied (I think, by you) as fixes pre-commit. However, there are some
new comments (or ones
that address the same lines, but in different ways). Is this mail to
log ~ what changes were done,
or is there anything actionable here on my side?

Best,
Zach

Thanks for this.
> Cheers,
>
> Alex
>
> > ---
> >   man2/madvise.2         | 90 +++++++++++++++++++++++++++++++++++++++++-
> >   man2/process_madvise.2 | 10 +++++
> >   2 files changed, 98 insertions(+), 2 deletions(-)
> >
> > diff --git a/man2/madvise.2 b/man2/madvise.2
> > index df3413cc8..b03fc731d 100644
> > --- a/man2/madvise.2
> > +++ b/man2/madvise.2
> > @@ -385,9 +385,10 @@ 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
> > @@ -400,6 +401,81 @@ 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
>
> Please use semantic line breaks.  In this case, I'd break after "pages".
>
> man-pages(7):
>     Use semantic newlines
>         In  the source of a manual page, new sentences should be started on new
>         lines, long sentences should be split into lines at clause breaks (com‐
>         mas, semicolons, colons, and so on), and long clauses should  be  split
>         at  phrase  boundaries.   This convention, sometimes known as "semantic
>         newlines", makes it easier to see the effect of  patches,  which  often
>         operate at the level of individual sentences, clauses, or phrases.
>
> > +memory range into Transparent Huge Pages (THPs).
> > +.B MADV_COLLAPSE
> > +operates on the current state of memory of the calling process and makes no
>
> Here I'd break after "and".
>
> > +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
>
> Break after "collapsing".
>
> > +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
>
> Break after "range".
>
> > +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
>
> Break before or after "and".
>
> > +in-place.
> > +Unmapped pages will have their data directly initialized to 0 in the new
>
> Break after "0".
>
> > +hugepage.
> > +However,
> > +for every eligible hugepage-aligned/sized region to be collapsed,
> > +at least one page must currently be backed by physical memory.
> > +.IP
> > +.BR MADV_COLLAPSE
>
> s/BR/B/
>
> > +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.
> > +.BR MADV_COLLAPSE
>
> s/BR/B/
>
> > +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
> > +.BR VM_NOHUGEPAGE
>
> s/BR/B/
>
> > +is still respected).
> > +.IP
> > +When the system has multiple NUMA nodes,
> > +the hugepage will be allocated from the node providing the most native
>
> Break after "from".
>
> > +pages.
> > +.IP
> > +If all hugepage-sized/aligned regions covered by the provided range were
>
> Prefer English rather than "/".
>
> > +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
>
> Break after "about".
>
> > +the memory.
> > +Also note that many failures might have occurred since the operation may
> > +continue to collapse in the event collapse of a single hugepage-sized/aligned
>
> Add some omitted "that" or something that will help readability to
> non-native-English readers.
>
> And break at a better place.
>
> > +region fails.
> > +.TP
> >   .BR MADV_DONTDUMP " (since Linux 3.4)"
> >   .\" commit 909af768e88867016f427264ae39d27a57b6a8ed
> >   .\" commit accb61fe7bb0f5c2a4102239e4981650f9048519
> > @@ -619,6 +695,11 @@ 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
> > @@ -716,6 +797,11 @@ 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
> > diff --git a/man2/process_madvise.2 b/man2/process_madvise.2
> > index 44d3b94e8..8b0ddccdd 100644
> > --- a/man2/process_madvise.2
> > +++ b/man2/process_madvise.2
> > @@ -73,6 +73,10 @@ argument is one of the following values:
> >   See
> >   .BR madvise (2).
> >   .TP
> > +.B MADV_COLLAPSE
> > +See
> > +.BR madvise (2).
> > +.TP
> >   .B MADV_PAGEOUT
> >   See
> >   .BR madvise (2).
> > @@ -173,6 +177,12 @@ The caller does not have permission to access the address space of the process
> >   .TP
> >   .B ESRCH
> >   The target process does not exist (i.e., it has terminated and been waited on).
> > +.PP
> > +See
> > +.BR madvise (2)
> > +for
> > +.IR advice -specific
> > +errors.
> >   .SH VERSIONS
> >   This system call first appeared in Linux 5.10.
> >   .\" commit ecb8ac8b1f146915aa6b96449b66dd48984caacc
>
> --
> <http://www.alejandro-colomar.es/>