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

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

On Fri, Dec 06, 2024 at 11:03:56AM +0000, Lorenzo Stoakes wrote:
> 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.

Actually, as discussed on IRC, I think your suggested 'replaced' is better, so:

"If the region maps memory pages, those mappings will be replaced".

Will address in v5.

>
> >
> > 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.
> > >