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Message-ID: <799f064f-8ca6-441d-81f4-94781bf5465a@lucifer.local>
Date: Wed, 4 Jun 2025 06:22:19 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Barry Song <v-songbaohua@...o.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
Jann Horn <jannh@...gle.com>, Lokesh Gidra <lokeshgidra@...gle.com>,
Tangquan Zheng <zhengtangquan@...o.com>
Subject: Re: [PATCH RFC v2] mm: use per_vma lock for MADV_DONTNEED
These notes are for Barry to adapt according to his needs for this series,
but FWIW agreed on both counts! :)
On Tue, Jun 03, 2025 at 01:17:10PM -0700, Suren Baghdasaryan wrote:
> On Tue, Jun 3, 2025 at 11:43 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > Hi Barry,
> >
> > As promised, I enclose a patch to give a sense of how I think we might
> > thread state through this operation.
> >
> > There's a todo on the untagged stuff so you can figure that out. This is
> > based on the v1 so it might not encompass everything you addressed in the
> > v2.
> >
> > Passing in madv_behavior to madvise_walk_vmas() twice kinda sucks, I
> > _despise_ the void *arg function ptr stuff there added just for the anon
> > vma name stuff (ughhh) so might be the only sensible way of threading
> > state.
> >
> > I don't need any attribution, so please use this patch as you see
> > fit/adapt/delete/do whatever with it, just an easier way for me to show the
> > idea!
> >
> > I did some very basic testing and it seems to work, but nothing deeper.
> >
> > Cheers, Lorenzo
> >
> > ----8<----
> > From ff4ba0115cb31a0630b6f8c02c68f11b3fb71f7a Mon Sep 17 00:00:00 2001
> > From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Date: Tue, 3 Jun 2025 18:22:55 +0100
> > Subject: [PATCH] mm/madvise: support VMA read locks for MADV_DONTNEED[_LOCKED]
> >
> > Refactor the madvise() code to retain state about the locking mode utilised
> > for traversing VMAs.
> >
> > Then use this mechanism to permit VMA locking to be done later in the
> > madvise() logic and also to allow altering of the locking mode to permit
> > falling back to an mmap read lock if required.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> > mm/madvise.c | 174 +++++++++++++++++++++++++++++++++++++--------------
> > 1 file changed, 127 insertions(+), 47 deletions(-)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 5f7a66a1617e..a3a6d73d0bd5 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -48,38 +48,19 @@ struct madvise_walk_private {
> > bool pageout;
> > };
> >
> > +enum madvise_lock_mode {
> > + MADVISE_NO_LOCK,
> > + MADVISE_MMAP_READ_LOCK,
> > + MADVISE_MMAP_WRITE_LOCK,
> > + MADVISE_VMA_READ_LOCK,
> > +};
> > +
> > struct madvise_behavior {
> > int behavior;
> > struct mmu_gather *tlb;
> > + enum madvise_lock_mode lock_mode;
> > };
> >
> > -/*
> > - * Any behaviour which results in changes to the vma->vm_flags needs to
> > - * take mmap_lock for writing. Others, which simply traverse vmas, need
> > - * to only take it for reading.
> > - */
> > -static int madvise_need_mmap_write(int behavior)
> > -{
> > - switch (behavior) {
> > - case MADV_REMOVE:
> > - case MADV_WILLNEED:
> > - case MADV_DONTNEED:
> > - case MADV_DONTNEED_LOCKED:
> > - case MADV_COLD:
> > - case MADV_PAGEOUT:
> > - case MADV_FREE:
> > - case MADV_POPULATE_READ:
> > - case MADV_POPULATE_WRITE:
> > - case MADV_COLLAPSE:
> > - case MADV_GUARD_INSTALL:
> > - case MADV_GUARD_REMOVE:
> > - return 0;
> > - default:
> > - /* be safe, default to 1. list exceptions explicitly */
> > - return 1;
> > - }
> > -}
> > -
> > #ifdef CONFIG_ANON_VMA_NAME
> > struct anon_vma_name *anon_vma_name_alloc(const char *name)
> > {
> > @@ -1486,6 +1467,43 @@ static bool process_madvise_remote_valid(int behavior)
> > }
> > }
> >
> > +/*
> > + * Try to acquire a VMA read lock if possible.
> > + *
> > + * We only support this lock over a single VMA, which the input range must
> > + * span.either partially or fully.
> > + *
> > + * This function always returns with an appropriate lock held. If a VMA read
> > + * lock could be acquired, we return the locked VMA.
> > + *
> > + * If a VMA read lock could not be acquired, we return NULL and expect caller to
>
> Worth mentioning that the function itself will fall back to taking
> mmap_read_lock in such a case.
>
> > + * fallback to mmap lock behaviour.
> > + */
> > +static struct vm_area_struct *try_vma_read_lock(struct mm_struct *mm,
> > + struct madvise_behavior *madv_behavior,
> > + unsigned long start, unsigned long end)
> > +{
> > + struct vm_area_struct *vma;
> > +
> > + if (!madv_behavior || madv_behavior->lock_mode != MADVISE_VMA_READ_LOCK)
>
> nit: I think it would be better to do this check before calling
> try_vma_read_lock(). IMHO it does not make sense to call
> try_vma_read_lock() when lock_mode != MADVISE_VMA_READ_LOCK. It also
> makes reading this function easier. The first time I looked at it and
> saw "return NULL" in one place that takes mmap_read_lock() and another
> place which returns the same NULL but does not take mmap_lock really
> confused me.
>
> > + return NULL;
> > +
> > + vma = lock_vma_under_rcu(mm, start);
> > + if (!vma)
> > + goto take_mmap_read_lock;
> > + /* We must span only a single VMA, uffd unsupported. */
> > + if (end > vma->vm_end || userfaultfd_armed(vma)) {
>
> vma->vm_end is not inclusive, so the above condition I think should be
> (end >= vma->vm_end || ...)
>
> > + vma_end_read(vma);
> > + goto take_mmap_read_lock;
> > + }
> > + return vma;
> > +
> > +take_mmap_read_lock:
> > + mmap_read_lock(mm);
> > + madv_behavior->lock_mode = MADVISE_MMAP_READ_LOCK;
> > + return NULL;
> > +}
> > +
> > /*
> > * Walk the vmas in range [start,end), and call the visit function on each one.
> > * The visit function will get start and end parameters that cover the overlap
> > @@ -1496,7 +1514,8 @@ static bool process_madvise_remote_valid(int behavior)
> > */
> > static
> > int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
> > - unsigned long end, void *arg,
> > + unsigned long end, struct madvise_behavior *madv_behavior,
> > + void *arg,
> > int (*visit)(struct vm_area_struct *vma,
> > struct vm_area_struct **prev, unsigned long start,
> > unsigned long end, void *arg))
> > @@ -1505,6 +1524,15 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
> > struct vm_area_struct *prev;
> > unsigned long tmp;
> > int unmapped_error = 0;
> > + int error;
> > +
> > + /* If VMA read lock supported, we apply advice to a single VMA only. */
> > + vma = try_vma_read_lock(mm, madv_behavior, start, end);
> > + if (vma) {
> > + error = visit(vma, &prev, start, end, arg);
> > + vma_end_read(vma);
> > + return error;
> > + }
> >
> > /*
> > * If the interval [start,end) covers some unmapped address
> > @@ -1516,8 +1544,6 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
> > prev = vma;
> >
> > for (;;) {
> > - int error;
> > -
> > /* Still start < end. */
> > if (!vma)
> > return -ENOMEM;
> > @@ -1598,34 +1624,86 @@ int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
> > if (end == start)
> > return 0;
> >
> > - return madvise_walk_vmas(mm, start, end, anon_name,
> > + return madvise_walk_vmas(mm, start, end, anon_name, NULL,
> > madvise_vma_anon_name);
> > }
> > #endif /* CONFIG_ANON_VMA_NAME */
> >
> > -static int madvise_lock(struct mm_struct *mm, int behavior)
> > +
> > +/*
> > + * Any behaviour which results in changes to the vma->vm_flags needs to
> > + * take mmap_lock for writing. Others, which simply traverse vmas, need
> > + * to only take it for reading.
> > + */
> > +static enum madvise_lock_mode get_lock_mode(struct madvise_behavior *madv_behavior)
> > {
> > + int behavior = madv_behavior->behavior;
> > +
> > if (is_memory_failure(behavior))
> > - return 0;
> > + return MADVISE_NO_LOCK;
> >
> > - if (madvise_need_mmap_write(behavior)) {
> > + switch (behavior) {
> > + case MADV_REMOVE:
> > + case MADV_WILLNEED:
> > + case MADV_COLD:
> > + case MADV_PAGEOUT:
> > + case MADV_FREE:
> > + case MADV_POPULATE_READ:
> > + case MADV_POPULATE_WRITE:
> > + case MADV_COLLAPSE:
> > + case MADV_GUARD_INSTALL:
> > + case MADV_GUARD_REMOVE:
> > + return MADVISE_MMAP_READ_LOCK;
> > + case MADV_DONTNEED:
> > + case MADV_DONTNEED_LOCKED:
> > + return MADVISE_VMA_READ_LOCK;
> > + default:
> > + return MADVISE_MMAP_WRITE_LOCK;
> > + }
> > +}
> > +
> > +static int madvise_lock(struct mm_struct *mm,
> > + struct madvise_behavior *madv_behavior)
> > +{
> > + enum madvise_lock_mode lock_mode = get_lock_mode(madv_behavior);
> > +
> > + switch (lock_mode) {
> > + case MADVISE_NO_LOCK:
> > + break;
> > + case MADVISE_MMAP_WRITE_LOCK:
> > if (mmap_write_lock_killable(mm))
> > return -EINTR;
> > - } else {
> > + break;
> > + case MADVISE_MMAP_READ_LOCK:
> > mmap_read_lock(mm);
> > + break;
> > + case MADVISE_VMA_READ_LOCK:
> > + /* We will acquire the lock per-VMA in madvise_walk_vmas(). */
> > + break;
> > }
> > +
> > + madv_behavior->lock_mode = lock_mode;
> > return 0;
> > }
> >
> > -static void madvise_unlock(struct mm_struct *mm, int behavior)
> > +static void madvise_unlock(struct mm_struct *mm,
> > + struct madvise_behavior *madv_behavior)
> > {
> > - if (is_memory_failure(behavior))
> > + switch (madv_behavior->lock_mode) {
> > + case MADVISE_NO_LOCK:
> > return;
> > -
> > - if (madvise_need_mmap_write(behavior))
> > + case MADVISE_MMAP_WRITE_LOCK:
> > mmap_write_unlock(mm);
> > - else
> > + break;
> > + case MADVISE_MMAP_READ_LOCK:
> > mmap_read_unlock(mm);
> > + break;
> > + case MADVISE_VMA_READ_LOCK:
> > + /* We will drop the lock per-VMA in madvise_walk_vmas(). */
> > + break;
> > + }
> > +
> > + madv_behavior->lock_mode = MADVISE_NO_LOCK;
> > }
> >
> > static bool madvise_batch_tlb_flush(int behavior)
> > @@ -1721,6 +1799,8 @@ static int madvise_do_behavior(struct mm_struct *mm,
> >
> > if (is_memory_failure(behavior))
> > return madvise_inject_error(behavior, start, start + len_in);
> > +
> > + // TODO: handle untagged stuff here...
> > start = untagged_addr(start); //untagged_addr_remote(mm, start);
> > end = start + PAGE_ALIGN(len_in);
> >
> > @@ -1729,7 +1809,7 @@ static int madvise_do_behavior(struct mm_struct *mm,
> > error = madvise_populate(mm, start, end, behavior);
> > else
> > error = madvise_walk_vmas(mm, start, end, madv_behavior,
> > - madvise_vma_behavior);
> > + madv_behavior, madvise_vma_behavior);
> > blk_finish_plug(&plug);
> > return error;
> > }
> > @@ -1817,13 +1897,13 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
> >
> > if (madvise_should_skip(start, len_in, behavior, &error))
> > return error;
> > - error = madvise_lock(mm, behavior);
> > + error = madvise_lock(mm, &madv_behavior);
> > if (error)
> > return error;
> > madvise_init_tlb(&madv_behavior, mm);
> > error = madvise_do_behavior(mm, start, len_in, &madv_behavior);
> > madvise_finish_tlb(&madv_behavior);
> > - madvise_unlock(mm, behavior);
> > + madvise_unlock(mm, &madv_behavior);
> >
> > return error;
> > }
> > @@ -1847,7 +1927,7 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
> >
> > total_len = iov_iter_count(iter);
> >
> > - ret = madvise_lock(mm, behavior);
> > + ret = madvise_lock(mm, &madv_behavior);
> > if (ret)
> > return ret;
> > madvise_init_tlb(&madv_behavior, mm);
> > @@ -1880,8 +1960,8 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
> >
> > /* Drop and reacquire lock to unwind race. */
> > madvise_finish_tlb(&madv_behavior);
> > - madvise_unlock(mm, behavior);
> > - ret = madvise_lock(mm, behavior);
> > + madvise_unlock(mm, &madv_behavior);
> > + ret = madvise_lock(mm, &madv_behavior);
> > if (ret)
> > goto out;
> > madvise_init_tlb(&madv_behavior, mm);
> > @@ -1892,7 +1972,7 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
> > iov_iter_advance(iter, iter_iov_len(iter));
> > }
> > madvise_finish_tlb(&madv_behavior);
> > - madvise_unlock(mm, behavior);
> > + madvise_unlock(mm, &madv_behavior);
> >
> > out:
> > ret = (total_len - iov_iter_count(iter)) ? : ret;
> > --
> > 2.49.0
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