Re: [PATCH man-pages v4] madvise.2: add MADV_GUARD_INSTALL, MADV_GUARD_REMOVE description

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Dec 05, 2024 at 09:43:09PM +0100, Vlastimil Babka wrote:
> On 12/5/24 19:09, Lorenzo Stoakes wrote:
> > On Thu, Dec 05, 2024 at 06:50:09PM +0100, Vlastimil Babka wrote:
> >> On 12/5/24 11:41, Lorenzo Stoakes wrote:
> >> > Lightweight guard region support has been added to Linux 6.13, which adds
> >> > MADV_GUARD_INSTALL and MADV_GUARD_REMOVE flags to the madvise() system
> >> > call. Therefore, update the manpage for madvise() and describe these
> >> > operations.
> >> >
> >> > Reviewed-by: Jann Horn <jannh@google.com>
> >> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> >> > ---
> >> > v4:
> >> > * Reference function chapters as per Alejandro.
> >> > * Minor rewording as per Alejandro.
> >> >
> >> > v3:
> >> > * Don't describe SIGSEGV as a fatal signal as per Jann.
> >> > https://lore.kernel.org/all/20241202165829.72121-1-lorenzo.stoakes@oracle.com
> >> >
> >> > v2:
> >> > * Updated to use semantic newlines as suggested by Alejandro.
> >> > * Avoided emboldening parens as suggested by Alejandro.
> >> > * One very minor grammatical fix.
> >> > https://lore.kernel.org/all/20241129155943.85215-1-lorenzo.stoakes@oracle.com
> >> >
> >> > v1:
> >> > https://lore.kernel.org/all/20241129093205.8664-1-lorenzo.stoakes@oracle.com
> >> >
> >> >  man/man2/madvise.2 | 93 ++++++++++++++++++++++++++++++++++++++++++++++
> >> >  1 file changed, 93 insertions(+)
> >> >
> >> > diff --git a/man/man2/madvise.2 b/man/man2/madvise.2
> >> > index 4f2210ee2..7d682fa40 100644
> >> > --- a/man/man2/madvise.2
> >> > +++ b/man/man2/madvise.2
> >> > @@ -676,6 +676,91 @@ or secret memory regions created using
> >> >  Note that with
> >> >  .BR MADV_POPULATE_WRITE ,
> >> >  the process can be killed at any moment when the system runs out of memory.
> >> > +.TP
> >> > +.BR MADV_GUARD_INSTALL " (since Linux 6.13)"
> >> > +Install a lightweight guard region into the range specified by
> >> > +.I addr
> >> > +and
> >> > +.IR size ,
> >> > +causing any read or write in the range to result in a
> >> > +.B SIGSEGV
> >> > +signal being raised.
> >> > +.IP
> >> > +If the region maps memory pages they will be cleared as part of the operation,
> >> > +though if
> >>
> >> Hm this reads a bit ambiguous. One could read it as the memory pages are
> >> being cleared, but it's the page tables.
> >
> > This was really hard to word, because you don't want to say unmapped, and saying
> > 'clearing page tables' or 'zapping' is clear to us but not necessarily to a
> > reader. 'Clearing mapping' makes it ambiguous vs. munmap(), etc. etc.
>
> Maybe saying "removed" instead of "cleared" would be better?

I was about to put more philosophical points here then realised we already have
a better solution... we can say 'the regions will become zero-fill-on-demand'.

Let me respin a v5 with this corrected. We will need to revisit once we
support file-backed pages.

>
> Anyway, I don't want to cause bikeshedding, so in any case:

No it's appreciated! Review should be critical so we can make this as good
as we can make it.

>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!

>
> > But you want to make it clear (no pun intended) that anon pages, if there's any
> > data, it's probably lost. So I think this is a distinction that doesn't matter.
> >
> > Will revisit once we support file-backed mappings.
> >