netdev - RE: [PATCH v6 rdma-next 1/6] RDMA/core: Create mmap database and cookie helper functions

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Date:   Mon, 29 Jul 2019 12:58:29 +0000
From:   Michal Kalderon <mkalderon@...vell.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        "galpress@...zon.com" <galpress@...zon.com>
CC:     Ariel Elior <aelior@...vell.com>,
        "dledford@...hat.com" <dledford@...hat.com>,
        "galpress@...zon.com" <galpress@...zon.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH v6 rdma-next 1/6] RDMA/core: Create mmap database and
 cookie helper functions

> From: linux-rdma-owner@...r.kernel.org <linux-rdma-
> owner@...r.kernel.org> On Behalf Of Jason Gunthorpe
> 
> > +	xa_lock(&ucontext->mmap_xa);
> > +	if (check_add_overflow(ucontext->mmap_xa_page,
> > +			       (u32)(length >> PAGE_SHIFT),
> > +			       &next_mmap_page))
> > +		goto err_unlock;
> 
> I still don't like that this algorithm latches into a permanent failure when the
> xa_page wraps.
> 
> It seems worth spending a bit more time here to tidy this.. Keep using the
> mmap_xa_page scheme, but instead do something like
> 
> alloc_cyclic_range():
> 
> while () {
>    // Find first empty element in a cyclic way
>    xa_page_first = mmap_xa_page;
>    xa_find(xa, &xa_page_first, U32_MAX, XA_FREE_MARK)
> 
>    // Is there a enough room to have the range?
>    if (check_add_overflow(xa_page_first, npages, &xa_page_end)) {
>       mmap_xa_page = 0;
>       continue;
>    }
> 
>    // See if the element before intersects
>    elm = xa_find(xa, &zero, xa_page_end, 0);
>    if (elm && intersects(xa_page_first, xa_page_last, elm->first, elm->last)) {
>       mmap_xa_page = elm->last + 1;
>       continue
>    }
> 
>    // xa_page_first -> xa_page_end should now be free
>    xa_insert(xa, xa_page_start, entry);
>    mmap_xa_page = xa_page_end + 1;
>    return xa_page_start;
> }
> 
> Approximately, please check it.
Gal & Jason, 

Coming back to the mmap_xa_page algorithm. I couldn't find some background on this. 
Why do you need the length to be represented in the mmap_xa_page ?  
Why not simply use xa_alloc_cyclic ( like in siw ) 
This is simply a key to a mmap object... 

Thanks,
Michal