Re: [PATCH v3 1/2] mm: vmalloc: Remove a global vmap_blocks xarray

Uladzislau Rezki
2023-03-29T17:50:59Z

On Wed, Mar 29, 2023 at 05:23:04PM +0100, Lorenzo Stoakes wrote:
> On Wed, Mar 29, 2023 at 05:01:11PM +0200, Uladzislau Rezki wrote:
> > Hello, Lorenzo!
> >
> > > > /*
> > > > - * 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?
> > >
> > > I like the general heft of your comment but I feel this could be spelled
> > > out a little more clearly, something like:-
> > >
> > > In order to have fast access to any vmap_block object associated with a
> > > specific address, we use a hash.
> > >
> > > Rather than waste space on defining a new hash table we take advantage
> > > of the fact we already have a static per-cpu array vmap_block_queue.
> > >
> > > This is already used for per-CPU access to the block queue, however we
> > > overload this to _also_ act as a vmap_block hash. The hash function is
> > > addr_to_vbq() which hashes on vb->va->va_start.
> > >
> > > This then uses per_cpu() to lookup the _index_ rather than the
> > > _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are
> > > indexed on the same key as the hash (vb->va->va_start).
> > >
> > > xarray read acceses are protected by RCU lock and inserts are protected
> > > by a spin lock so there is no risk of a race here.
> > >
> > /*
> > * In order to fast access to any "vmap_block" associated with a
> > * specific address, we use a hash.
> > *
> > * A per-cpu vmap_block_queue is used in both ways, to serialize
> > * an access to free block chains among CPUs(alloc path) and it
> > * also acts as a vmap_block hash(alloc/free paths). It means we
> > * overload it, since we already have the per-cpu array which is
> > * used as a hash table.
>
> Nit - it may be worth highlighting that when used as a hash it the 'cpu' is
> not in fact a cpu but rather a hash key.
>
> E.g. just add on the end of this something like:-
>
> When used as a hash table the 'cpu' passed to per_cpu is not actually a CPU
> but rather the hash key.
>
> > *
> > * A hash function is addr_to_vbq() which hashes any address to
> > * a specific index(in a hash) it belongs to. This then uses a
> > * per_cpu() macro to access the array with specific index.
>
> May need a tweak if you are happy with my review that we can simply have a
> helper that returns the xarray in which case we won't necessary have this
> function :) but depends of course on how the respin looks!
>
> > *
> > * 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.
> > *
> > * This technique allows almost remove a lock-contention in locking
> > * primitives which protect insert/remove operations.
>
> This sentence is a little confusing, perhaps rephrase a little:-
>
> This technique almost always avoids lock contention on insert/remove,
> however the xarray spinlock protects against any contention that remains.
>
> > */
> > Are you find with it?
>
> Other than the small nits above (sorry!) it seems fine! Thanks for
> updating, much appreciated :)
>
Good. Made the changes. I will upload a new vX patch. Everything
that makes it more clear for readers is worth to do :)