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

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Message-ID: <MN2PR18MB31823CA6395E3DCBC16B2647A1DD0@MN2PR18MB3182.namprd18.prod.outlook.com>
Date:   Mon, 29 Jul 2019 14:07:23 +0000
From:   Michal Kalderon <mkalderon@...vell.com>
To:     Gal Pressman <galpress@...zon.com>, Jason Gunthorpe <jgg@...pe.ca>
CC:     Ariel Elior <aelior@...vell.com>,
        "dledford@...hat.com" <dledford@...hat.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: Gal Pressman <galpress@...zon.com>
> Sent: Monday, July 29, 2019 4:54 PM
> 
> On 29/07/2019 15:58, Michal Kalderon wrote:
> >> 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...
> 
> The intention was that the entry would "occupy" number of xarray elements
> according to its size (in pages). It wasn't initially like this, but IIRC this was
> preferred by Jason.

Thanks, so Jason, if we're now freeing the objects, can we simply us xa_alloc_cyclic instead? 
Thanks,
Michal