Re: [Xen-devel] [PATCH v4 2/4] x86/mem_sharing: copy a page_lock version to be internal to memshr

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.05.19 at 15:43, <tamas@xxxxxxxxxxxxx> wrote:
> On Fri, May 3, 2019 at 2:27 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>
>> >>> On 03.05.19 at 00:13, <tamas@xxxxxxxxxxxxx> wrote:
>> > --- a/xen/arch/x86/mm/mem_sharing.c
>> > +++ b/xen/arch/x86/mm/mem_sharing.c
>> > @@ -112,13 +112,48 @@ static inline void page_sharing_dispose(struct 
>> > page_info *page)
>> >
>> >  #endif /* MEM_SHARING_AUDIT */
>> >
>> > -static inline int mem_sharing_page_lock(struct page_info *pg)
>> > +/*
>> > + * Private implementations of page_lock/unlock to bypass PV-only
>> > + * sanity checks not applicable to mem-sharing.
>> > + */
>> > +static inline bool _page_lock(struct page_info *page)
>> >  {
>> > -    int rc;
>> > +    unsigned long x, nx;
>> > +
>> > +    do {
>> > +        while ( (x = page->u.inuse.type_info) & PGT_locked )
>> > +            cpu_relax();
>> > +        nx = x + (1 | PGT_locked);
>> > +        if ( !(x & PGT_validated) ||
>> > +             !(x & PGT_count_mask) ||
>> > +             !(nx & PGT_count_mask) )
>> > +            return false;
>>
>> Just for my own understanding: Did you verify that the PGT_validated
>> check is indeed needed here, or did you copy it "just in case"? In the
>> latter case a comment may be worthwhile.
> 
> This is an exact copy of page_lock, sans the asserts that break it
> from mem_sharing. I didn't investigate which of these flags are
> necessary for mem_sharing. Frankly, I don't fully understand their
> meaning and I haven't came across documentation about it yet. I can
> certainly add a comment saying TODO: figure out which of these flags
> are actually needed.

Yes something along these lines is what I'm after. But "these flags"
really is just PGT_validated. There's no question the PGT_locked
and PGT_count_mask operations need to remain as they are.

Jan



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