linux-kernel - Re: [PATCH v3 1/2] mm: vmalloc: Remove a global vmap

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Date:   Tue, 28 Mar 2023 18:37:55 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, Baoquan He <bhe@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v3 1/2] mm: vmalloc: Remove a global vmap_blocks xarray

> >  /*
> > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block
> > - * in the free path. Could get rid of this if we change the API to return a
> > - * "cookie" from alloc, to be passed to free. But no big deal yet.
> > + * In order to fast access to any "vmap_block" associated with a
> > + * specific address, we store them into a per-cpu xarray. A hash
> > + * function is addr_to_vbq() whereas a key is a vb->va->va_start
> > + * value.
> > + *
> > + * Please note, a vmap_block_queue, which is a per-cpu, is not
> > + * serialized by a raw_smp_processor_id() current CPU, instead
> > + * it is chosen based on a CPU-index it belongs to, i.e. it is
> > + * a hash-table.
> > + *
> > + * An example:
> > + *
> > + *  CPU_1  CPU_2  CPU_0
> > + *    |      |      |
> > + *    V      V      V
> > + * 0     10     20     30     40     50     60
> > + * |------|------|------|------|------|------|...<vmap address space>
> > + *   CPU0   CPU1   CPU2   CPU0   CPU1   CPU2
> > + *
> > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus
> > + *   it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus
> > + *   it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock;
> > + *
> > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus
> > + *   it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock.
> >   */
> 
> OK so if I understand this correctly, you're overloading the per-CPU
> vmap_block_queue array to use as a simple hash based on the address and
> relying on the xa_lock() in xa_insert() to serialise in case of contention?
> 
Sorry i missed your question. You correctly understood what i am doing.

Basically, we can associate any address with an index in per-cpu-array.
Since a CPU pre-allocates a fixed block size, which is a VMAP_BLOCK_SIZE, 
we can map any address within this block to a certain index or i call
it a specific CPU zone it belongs to.

If we want fully serialize it we have to allocate a new vmap block in
CPU owner zone. According to ASCII picture, for CPU0 it is 0-20, 30-40
addresses. In fact, even though it would be "fully" serialized, in practise
id does not give a visible performance. So this is not needed and it
has extra drawbacks.

--
Uladzislau Rezki