From: Peng Zhang <zhangpeng.00@bytedance.com>
To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
Peng Zhang <zhangpeng.00@bytedance.com>,
corbet@lwn.net, akpm@linux-foundation.org, willy@infradead.org,
brauner@kernel.org, surenb@google.com,
michael.christie@oracle.com, peterz@infradead.org,
mathieu.desnoyers@efficios.com, npiggin@gmail.com,
avagin@gmail.com, linux-mm@kvack.org, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 6/6] fork: Use __mt_dup() to duplicate maple tree in dup_mmap()
Date: Mon, 18 Sep 2023 21:14:42 +0800 [thread overview]
Message-ID: <7f0db16b-609c-7504-bd5b-52a76e0200b3@bytedance.com> (raw)
In-Reply-To: <20230915200040.ebrri6zy3uccndx2@revolver>
在 2023/9/16 04:00, Liam R. Howlett 写道:
> * Peng Zhang <zhangpeng.00@bytedance.com> [230915 06:57]:
>>
>>
>
> ...
>
>>>>> + if (unlikely(retval))
>>>>> goto out;
>>>>> mt_clear_in_rcu(vmi.mas.tree);
>>>>> - for_each_vma(old_vmi, mpnt) {
>>>>> + for_each_vma(vmi, mpnt) {
>>>>> struct file *file;
>>>>> vma_start_write(mpnt);
>>>>> if (mpnt->vm_flags & VM_DONTCOPY) {
>>>>> vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
>>>>> +
>>>>> + /*
>>>>> + * Since the new tree is exactly the same as the old one,
>>>>> + * we need to remove the unneeded VMAs.
>>>>> + */
>>>>> + mas_store(&vmi.mas, NULL);
>>>>> +
>>>>> + /*
>>>>> + * Even removing an entry may require memory allocation,
>>>>> + * and if removal fails, we use XA_ZERO_ENTRY to mark
>>>>> + * from which VMA it failed. The case of encountering
>>>>> + * XA_ZERO_ENTRY will be handled in exit_mmap().
>>>>> + */
>>>>> + if (unlikely(mas_is_err(&vmi.mas))) {
>>>>> + retval = xa_err(vmi.mas.node);
>>>>> + mas_reset(&vmi.mas);
>>>>> + if (mas_find(&vmi.mas, ULONG_MAX))
>>>>> + mas_store(&vmi.mas, XA_ZERO_ENTRY);
>>>>> + goto loop_out;
>>>>> + }
>>>>> +
>>>>
>>>> Storing NULL may need extra space as you noted, so we need to be careful
>>>> what happens if we don't have that space. We should have a testcase to
>>>> test this scenario.
>>>>
>>>> mas_store_gfp() should be used with GFP_KERNEL. The VMAs use GFP_KERNEL
>>>> in this function, see vm_area_dup().
>>>>
>>>> Don't use the exit_mmap() path to undo a failed fork. You've added
>>>> checks and complications to the exit path for all tasks in the very
>>>> unlikely event that we run out of memory when we hit a very unlikely
>>>> VM_DONTCOPY flag.
>>>>
>>>> I see the issue with having a portion of the tree with new VMAs that are
>>>> accounted and a portion of the tree that has old VMAs that should not be
>>>> looked at. It was clever to use the XA_ZERO_ENTRY as a stop point, but
>>>> we cannot add that complication to the exit path and then there is the
>>>> OOM race to worry about (maybe, I am not sure since this MM isn't
>>>> active yet).
>>> I encountered some errors after implementing the scheme you mentioned
>>> below.
>
> What were the errors? Maybe I missed something or there is another way.
I found the cause of the problem and fixed it, tested the error path and
it seems to be working fine now.
The reason is that "free_pgd_range(tlb, addr, vma->vm_end,floor, next?
next->vm_start: ceiling);" in free_pgtables() does not free all page
tables due to the existence of the last false VMA. I've fixed it.
Thanks.
>
>>> This would also clutter fork.c and mmap.c, as some internal
>>> functions would need to be made global.
>
> Could it not be a new function in mm/mmap.c and added to mm/internal.h
> that does the accounting and VMA freeing from [0 - vma->vm_start)?
>
> Maybe we could use it in the other areas that do this sort of work?
> do_vmi_align_munmap() does something similar to what we need after the
> "Point of no return".
>
>>>
>>> I thought of another way to put everything into maple tree. In non-RCU
>>> mode, we can remove the last half of the tree without allocating any
>>> memory. This requires modifications to the internal implementation of
>>> mas_store().
>>> Then remove the second half of the tree like this:
>>>
>>> mas.index = 0;
>> Sorry, typo.
>> Change to: mas.index = vma->start
>>> mas.last = ULONGN_MAX;
>>> mas_store(&mas, NULL).
>
> Well, we know we are not in RCU mode here, but I am concerned about this
> going poorly.
>
>>
>>>
>>> At least in non-RCU mode, we can do this, since we only need to merge
>>> some nodes, or move some items to adjacent nodes.
>>> However, this will increase the workload significantly.
>
> In the unlikely event of an issue allocating memory, this would be
> unwelcome. If we can avoid it, it would be best. I don't mind being
> slow in error paths, but a significant workload would be rather bad on
> an overloaded system.
>
>>>
>>>>
>>>> Using what is done in exit_mmap() and do_vmi_align_munmap() as a
>>>> prototype, we can do something like the *untested* code below:
>>>>
>>>> if (unlikely(mas_is_err(&vmi.mas))) {
>>>> unsigned long max = vmi.index;
>>>>
>>>> retval = xa_err(vmi.mas.node);
>>>> mas_set(&vmi.mas, 0);
>>>> tmp = mas_find(&vmi.mas, ULONG_MAX);
>>>> if (tmp) { /* Not the first VMA failed */
>>>> unsigned long nr_accounted = 0;
>>>>
>>>> unmap_region(mm, &vmi.mas, vma, NULL, mpnt, 0, max, max,
>>>> true);
>>>> do {
>>>> if (vma->vm_flags & VM_ACCOUNT)
>>>> nr_accounted += vma_pages(vma);
>>>> remove_vma(vma, true);
>>>> cond_resched();
>>>> vma = mas_find(&vmi.mas, max - 1);
>>>> } while (vma != NULL);
>>>>
>>>> vm_unacct_memory(nr_accounted);
>>>> }
>>>> __mt_destroy(&mm->mm_mt);
>>>> goto loop_out;
>>>> }
>>>>
>>>> Once exit_mmap() is called, the check for OOM (no vma) will catch that
>>>> nothing is left to do.
>>>>
>>>> It might be worth making an inline function to do this to keep the fork
>>>> code clean. We should test this by detecting a specific task name and
>>>> returning a failure at a given interval:
>>>>
>>>> if (!strcmp(current->comm, "fork_test") {
>>>> ...
>>>> }
>>>>
> ...
>
>
> Thanks,
> Liam
>
next prev parent reply other threads:[~2023-09-18 13:14 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-30 12:56 [PATCH v2 0/6] Introduce __mt_dup() to improve the performance of fork() Peng Zhang
2023-08-30 12:56 ` [PATCH v2 1/6] maple_tree: Add two helpers Peng Zhang
2023-09-07 20:13 ` Liam R. Howlett
2023-09-08 2:45 ` Peng Zhang
2023-08-30 12:56 ` [PATCH v2 2/6] maple_tree: Introduce interfaces __mt_dup() and mtree_dup() Peng Zhang
2023-09-07 20:13 ` Liam R. Howlett
2023-09-08 9:26 ` Peng Zhang
2023-09-08 16:05 ` Liam R. Howlett
2023-09-11 12:59 ` Peng Zhang
2023-09-11 13:36 ` Liam R. Howlett
2023-08-30 12:56 ` [PATCH v2 3/6] maple_tree: Add test for mtree_dup() Peng Zhang
2023-09-07 20:13 ` Liam R. Howlett
2023-09-08 9:38 ` Peng Zhang
2023-09-25 4:06 ` Peng Zhang
2023-09-25 7:44 ` Liam R. Howlett
2023-09-25 8:30 ` Peng Zhang
2023-08-30 12:56 ` [PATCH v2 4/6] maple_tree: Skip other tests when BENCH is enabled Peng Zhang
2023-08-30 12:56 ` [PATCH v2 5/6] maple_tree: Update check_forking() and bench_forking() Peng Zhang
2023-08-31 13:40 ` kernel test robot
2023-09-01 10:58 ` Peng Zhang
2023-09-07 18:03 ` Liam R. Howlett
2023-09-07 18:16 ` Matthew Wilcox
2023-09-08 9:47 ` Peng Zhang
2023-09-07 20:14 ` Liam R. Howlett
2023-08-30 12:56 ` [PATCH v2 6/6] fork: Use __mt_dup() to duplicate maple tree in dup_mmap() Peng Zhang
2023-09-07 20:14 ` Liam R. Howlett
2023-09-08 9:58 ` Peng Zhang
2023-09-08 16:07 ` Liam R. Howlett
2023-09-15 10:51 ` Peng Zhang
2023-09-15 10:56 ` Peng Zhang
2023-09-15 20:00 ` Liam R. Howlett
2023-09-18 13:14 ` Peng Zhang [this message]
2023-09-18 17:59 ` Liam R. Howlett
2023-08-30 13:05 ` [PATCH v2 0/6] Introduce __mt_dup() to improve the performance of fork() Peng Zhang
2023-09-07 20:19 ` Liam R. Howlett
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