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Date: Thu, 5 Oct 2023 23:20:19 +0800
From: Yajun Deng <yajun.deng@...ux.dev>
To: Mike Rapoport <rppt@...nel.org>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] memblock: don't run loop in memblock_add_range() twice
On 2023/10/5 13:19, Mike Rapoport wrote:
> On Wed, Oct 04, 2023 at 12:30:45AM +0800, Yajun Deng wrote:
>> There is round twice in memblock_add_range(). The first counts the number
>> of regions needed to accommodate the new area. The second actually inserts
>> them. But the first round isn't really needed, we just need to check the
>> counts before inserting them.
>>
>> Check the count before memblock_insert_region. If the count is equal to
>> the maximum, it needs to resize the array. Otherwise, insert it directly.
>>
>> Also, there is a nested call here, we need to reserve the current array
>> immediately if slab is unavailable.
> I presume this fixes a bug you found in v2, but are you sure it'll _never_
> explode on a machine with different memory layout and different sequence of
> memblock_reservee() calls?
Not really. It has become complex. Because it has to deal with the
nested call.
I think v1 is the best solution if you accept it. It doesn't need to
deal with the nested call. It would be safer.
I don't think we must do memblock_reserve() in memblock_double_array().
These parameters 'addr' and
'new_alloc_size' in memblock_reserve come from memblock_find_in_range().
Since we can handle these
parameters out of memblock_find_in_range(), we can also handle 'addr'
and 'new_alloc_size' out of
memblock_double_array().
> I don't see this micro-optimization is worth the churn and potential bugs.
> NAK.
There are many handouts that tell people it needs to run twice in
memblock_add_range().
I think it's time to change this. I'm trying to tell people that running
twice is unnecessary.
Like v1, it just needs to check the count and handle memblock_reserve
out of memblock_double_array.
>
>> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
>> ---
>> v3: reserve the current array immediately if slab is unavailable.
>> v2: remove the changes of memblock_double_array.
>> v1: https://lore.kernel.org/all/20230927013752.2515238-1-yajun.deng@linux.dev/
>> ---
>> mm/memblock.c | 93 +++++++++++++++++++++++----------------------------
>> 1 file changed, 41 insertions(+), 52 deletions(-)
>>
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index 5a88d6d24d79..71449c0b8bc8 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -588,11 +588,12 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>> phys_addr_t base, phys_addr_t size,
>> int nid, enum memblock_flags flags)
>> {
>> - bool insert = false;
>> phys_addr_t obase = base;
>> phys_addr_t end = base + memblock_cap_size(base, &size);
>> - int idx, nr_new, start_rgn = -1, end_rgn;
>> + int idx, start_rgn = -1, end_rgn;
>> struct memblock_region *rgn;
>> + int use_slab = slab_is_available();
>> + unsigned long ocnt = type->cnt;
>>
>> if (!size)
>> return 0;
>> @@ -608,25 +609,6 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>> return 0;
>> }
>>
>> - /*
>> - * The worst case is when new range overlaps all existing regions,
>> - * then we'll need type->cnt + 1 empty regions in @type. So if
>> - * type->cnt * 2 + 1 is less than or equal to type->max, we know
>> - * that there is enough empty regions in @type, and we can insert
>> - * regions directly.
>> - */
>> - if (type->cnt * 2 + 1 <= type->max)
>> - insert = true;
>> -
>> -repeat:
>> - /*
>> - * The following is executed twice. Once with %false @insert and
>> - * then with %true. The first counts the number of regions needed
>> - * to accommodate the new area. The second actually inserts them.
>> - */
>> - base = obase;
>> - nr_new = 0;
>> -
>> for_each_memblock_type(idx, type, rgn) {
>> phys_addr_t rbase = rgn->base;
>> phys_addr_t rend = rbase + rgn->size;
>> @@ -644,15 +626,30 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>> WARN_ON(nid != memblock_get_region_node(rgn));
>> #endif
>> WARN_ON(flags != rgn->flags);
>> - nr_new++;
>> - if (insert) {
>> - if (start_rgn == -1)
>> - start_rgn = idx;
>> - end_rgn = idx + 1;
>> - memblock_insert_region(type, idx++, base,
>> - rbase - base, nid,
>> - flags);
>> +
>> + /*
>> + * If type->cnt is equal to type->max, it means there's
>> + * not enough empty region and the array needs to be
>> + * resized. Otherwise, insert it directly.
>> + *
>> + * If slab is unavailable, it means a new array was reserved
>> + * in memblock_double_array. There is a nested call here, We
>> + * need to reserve the current array now if its type is
>> + * reserved.
>> + */
>> + if (type->cnt == type->max) {
>> + if (memblock_double_array(type, obase, size))
>> + return -ENOMEM;
>> + else if (!use_slab && type == &memblock.reserved)
>> + return memblock_reserve(obase, size);
>> }
>> +
>> + if (start_rgn == -1)
>> + start_rgn = idx;
>> + end_rgn = idx + 1;
>> + memblock_insert_region(type, idx++, base,
>> + rbase - base, nid,
>> + flags);
>> }
>> /* area below @rend is dealt with, forget about it */
>> base = min(rend, end);
>> @@ -660,33 +657,25 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>>
>> /* insert the remaining portion */
>> if (base < end) {
>> - nr_new++;
>> - if (insert) {
>> - if (start_rgn == -1)
>> - start_rgn = idx;
>> - end_rgn = idx + 1;
>> - memblock_insert_region(type, idx, base, end - base,
>> - nid, flags);
>> +
>> + if (type->cnt == type->max) {
>> + if (memblock_double_array(type, obase, size))
>> + return -ENOMEM;
>> + else if (!use_slab && type == &memblock.reserved)
>> + return memblock_reserve(obase, size);
>> }
>> - }
>>
>> - if (!nr_new)
>> - return 0;
>> + if (start_rgn == -1)
>> + start_rgn = idx;
>> + end_rgn = idx + 1;
>> + memblock_insert_region(type, idx, base, end - base,
>> + nid, flags);
>> + }
>>
>> - /*
>> - * If this was the first round, resize array and repeat for actual
>> - * insertions; otherwise, merge and return.
>> - */
>> - if (!insert) {
>> - while (type->cnt + nr_new > type->max)
>> - if (memblock_double_array(type, obase, size) < 0)
>> - return -ENOMEM;
>> - insert = true;
>> - goto repeat;
>> - } else {
>> + if (ocnt != type->cnt)
>> memblock_merge_regions(type, start_rgn, end_rgn);
>> - return 0;
>> - }
>> +
>> + return 0;
>> }
>>
>> /**
>> --
>> 2.25.1
>>
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