linux-kernel - Re: [PATCH v4 07/66] mm: Add VMA iterator

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Message-ID: <1d0b51de-59d0-c3f1-3136-4dabc2a873d1@suse.cz>
Date:   Fri, 10 Dec 2021 16:08:33 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Song Liu <songliubraving@...com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michel Lespinasse <walken.cr@...il.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Minchan Kim <minchan@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>
Subject: Re: [PATCH v4 07/66] mm: Add VMA iterator

On 12/10/21 03:02, Liam Howlett wrote:
> 
> vma_find() uses mas_find() which was created to implement find_vma().
> As a replacement, the search looks for an entry at the address and if
> nothing exists, it will continue the search upwards.  The result is that
> the first entry can be found at the address passed.  Every subsequent
> call to vma_find() would search from the end of the previous range - as
> saved in the maple state, or the vma iterator in this case.
> 
> mas_next(), however is more of a traditional linked list operation that
> finds the next entry _after_ the one containing the index in the maple
> state.  The only difference is on the start when the maple state is not
> currently pointing at an entry at all (the node is set to MAS_START).
> 
> mas_find() can be thought of as:
> 
> entry = mas_walk();
> if (!entry)
> 	entry = mas_next_entry();
> 
> return entry;
> 
> 
> mas_next can be though to as:
> 
> if (mas_is_start())
> 	mas_walk();
> 
> return mas_next_entry();
> 
> 
> Matthew uses mas_find() for his implementation of the vma iterator so
> that the first entry is not skipped.

Yeah, but if vma_next() is going to replace the cases where we already have
a vma and use vma->vm_next to get the next one, then mas_next() would be a
better fit?

Do I understand correctly that e.g. after a mas_pause(), vma_next() done via
max_next() might return the same vma again, while vma_prev() will not, and
vma_next() implemented by mas_next() also wouldn't? Isn't that unexpected
semantics?

>> 
>> > +}
>> > +
>> > +static inline struct vm_area_struct *vma_prev(struct vma_iterator *vmi)
>> > +{
>> > +	return mas_prev(&vmi->mas, 0);
>> > +}
>> > +
>> > +static inline unsigned long vma_iter_addr(struct vma_iterator *vmi)
>> > +{
>> > +	return vmi->mas.index;
>> > +}
>> > +
>> > +#define for_each_vma(vmi, vma)		while ((vma = vma_next(&vmi)) != NULL)
>> > +