linux-kernel - Re: [PATCH 4/5] mm: Introduce zap_details.zap

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Message-ID: <YTDkQMLhIfIVRGoG@t490s>
Date:   Thu, 2 Sep 2021 10:48:32 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yang Shi <shy828301@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Hugh Dickins <hughd@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH 4/5] mm: Introduce zap_details.zap_flags

On Thu, Sep 02, 2021 at 09:28:42AM +0200, David Hildenbrand wrote:
> On 01.09.21 22:57, Peter Xu wrote:
> > Instead of trying to introduce one variable for every new zap_details fields,
> > let's introduce a flag so that it can start to encode true/false informations.
> > 
> > Let's start to use this flag first to clean up the only check_mapping variable.
> > Firstly, the name "check_mapping" implies this is a "boolean", but actually it
> > stores the mapping inside, just in a way that it won't be set if we don't want
> > to check the mapping.
> > 
> > To make things clearer, introduce the 1st zap flag ZAP_FLAG_CHECK_MAPPING, so
> > that we only check against the mapping if this bit set.  At the same time, we
> > can rename check_mapping into zap_mapping and set it always.
> > 
> > Since at it, introduce another helper zap_check_mapping_skip() and use it in
> > zap_pte_range() properly.
> > 
> > Some old comments have been removed in zap_pte_range() because they're
> > duplicated, and since now we're with ZAP_FLAG_CHECK_MAPPING flag, it'll be very
> > easy to grep this information by simply grepping the flag.
> > 
> > It'll also make life easier when we want to e.g. pass in zap_flags into the
> > callers like unmap_mapping_pages() (instead of adding new booleans besides the
> > even_cows parameter).
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >   include/linux/mm.h | 19 ++++++++++++++++++-
> >   mm/memory.c        | 34 ++++++++++------------------------
> >   2 files changed, 28 insertions(+), 25 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 69259229f090..fcbc1c4f8e8e 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1716,14 +1716,31 @@ static inline bool can_do_mlock(void) { return false; }
> >   extern int user_shm_lock(size_t, struct ucounts *);
> >   extern void user_shm_unlock(size_t, struct ucounts *);
> > +/* Whether to check page->mapping when zapping */
> > +#define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
> 
> So we want to go full way, like:
> 
> typedef int __bitwise zap_flags_t;
> 
> #define  ZAP_FLAG_CHECK_MAPPING		((__force zap_flags_t)BIT(0))

Sure.

> 
> > +
> >   /*
> >    * Parameter block passed down to zap_pte_range in exceptional cases.
> >    */
> >   struct zap_details {
> > -	struct address_space *check_mapping;	/* Check page->mapping if set */
> > +	struct address_space *zap_mapping;
> >   	struct page *single_page;		/* Locked page to be unmapped */
> > +	unsigned long zap_flags;
> 
> Why call it "zap_*" if everything in the structure is related to zapping?
> IOW, simply "mapping", "flags" would be good enough.

Not sure if it's a good habit or bad - it's just for tagging system to be able
to identify other "mapping" variables, or a simple grep with the name.  So I
normally prefix fields with some special wording to avoid collisions.

> 
> >   };
> > +/* Return true if skip zapping this page, false otherwise */
> > +static inline bool
> > +zap_skip_check_mapping(struct zap_details *details, struct page *page)
> > +{
> > +	if (!details || !page)
> > +		return false;
> > +
> > +	if (!(details->zap_flags & ZAP_FLAG_CHECK_MAPPING))
> > +		return false;
> > +
> > +	return details->zap_mapping != page_rmapping(page);
> > +}
> 
> I'm confused, why isn't "!details->zap_mapping" vs. "details->zap_mapping"
> sufficient? I can see that you may need flags for other purposes (next
> patch), but why do we need it here?
> 
> Factoring it out into this helper is a nice cleanup, though. But I'd just
> not introduce ZAP_FLAG_CHECK_MAPPING.

Yes I think it's okay. I wanted to separate them as they're fundamentall two
things to me.  Example: what if the mapping we want to check is NULL itself
(remove private pages only; though it may not have a real user at least so
far)?  In that case one variable won't be able to cover it.

But indeed Matthew raised similar comment, so it seems to be a common
preference.  No strong opinion on my side, let me coordinate with it.

Thanks for looking,

-- 
Peter Xu