On Tuesday, 30 March 2021 2:42:34 PM AEDT John Hubbard wrote:
> On 3/29/21 5:38 PM, Alistair Popple wrote:
> > request_free_mem_region() is used to find an empty range of physical
> > addresses for hotplugging ZONE_DEVICE memory. It does this by iterating
> > over the range of possible addresses using region_intersects() to see if
> > the range is free.
> >
> > region_intersects() obtains a read lock before walking the resource tree
> > to protect against concurrent changes. However it drops the lock prior
> > to returning. This means by the time request_mem_region() is called in
> > request_free_mem_region() another thread may have already reserved the
> > requested region resulting in unexpected failures and a message in the
> > kernel log from hitting this condition:
> >
> > /*
> > * mm/hmm.c reserves physical addresses which then
> > * become unavailable to other users. Conflicts are
> > * not expected. Warn to aid debugging if encountered.
> > */
> > if (conflict->desc == IORES_DESC_DEVICE_PRIVATE_MEMORY) {
> > pr_warn("Unaddressable device %s %pR conflicts with %pR",
> > conflict->name, conflict, res);
> >
> > To fix this create versions of region_intersects() and
> > request_mem_region() that allow the caller to take the appropriate lock
> > such that it may be held over the required calls.
> >
> > Instead of creating another version of devm_request_mem_region() that
> > doesn't take the lock open-code it to allow the caller to pre-allocate
> > the required memory prior to taking the lock.
> >
> > Fixes: 0c385190392d8 ("resource: add a not device managed
request_free_mem_region variant")
> > Fixes: 0092908d16c60 ("mm: factor out a devm_request_free_mem_region
helper")
>
> Hi Alistair!
>
> The above "Fixes:" tag looks wrong to me, because that commit did not create
> the broken locking that this patch fixes. Therefore, I think that particular
line
> should be removed from the commit description.
Right, the last "Fixes:" tag is the origin of the bug but the refactoring into
different functions and files made it non-obvious how this patch was related.
Happy to drop these though.
> Another note below:
>
> > Fixes: 4ef589dc9b10c ("mm/hmm/devmem: device memory hotplug using
ZONE_DEVICE")
> > Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
> > Acked-by: Balbir Singh <bsingharora@xxxxxxxxx>
> > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> >
> > ---
> >
> > Hi Andrew,
> >
> > This fixes a boot issue reported by the kernel test robot with the
> > previous version of the patch on x86 with CONFIG_IO_STRICT_DEVMEM=y.
> > This was due to the platform specific implementation of
> > devmem_is_allowed() creating a recursive lock which I missed. I notice
> > you have put v2 in mmtom so apologies for the churn but can you please
> > use this version instead? Thanks.
> >
> > - Alistair
> > ---
> > kernel/resource.c | 142 ++++++++++++++++++++++++++++++----------------
> > 1 file changed, 92 insertions(+), 50 deletions(-)
> >
> > diff --git a/kernel/resource.c b/kernel/resource.c
> > index 627e61b0c124..7061b9f903ca 100644
> > --- a/kernel/resource.c
> > +++ b/kernel/resource.c
> > @@ -523,6 +523,34 @@ int __weak page_is_ram(unsigned long pfn)
> > }
> > EXPORT_SYMBOL_GPL(page_is_ram);
> >
> > +static int __region_intersects(resource_size_t start, size_t size,
> > + unsigned long flags, unsigned long desc)
> > +{
> > + struct resource res;
> > + int type = 0; int other = 0;
> > + struct resource *p;
> > +
> > + res.start = start;
> > + res.end = start + size - 1;
> > +
> > + for (p = iomem_resource.child; p ; p = p->sibling) {
> > + bool is_type = (((p->flags & flags) == flags) &&
> > + ((desc == IORES_DESC_NONE) ||
> > + (desc == p->desc)));
> > +
> > + if (resource_overlaps(p, &res))
> > + is_type ? type++ : other++;
> > + }
> > +
> > + if (type == 0)
> > + return REGION_DISJOINT;
> > +
> > + if (other == 0)
> > + return REGION_INTERSECTS;
> > +
> > + return REGION_MIXED;
> > +}
> > +
> > /**
> > * region_intersects() - determine intersection of region with known
resources
> > * @start: region start address
> > @@ -546,31 +574,12 @@ EXPORT_SYMBOL_GPL(page_is_ram);
> > int region_intersects(resource_size_t start, size_t size, unsigned long
flags,
> > unsigned long desc)
> > {
> > - struct resource res;
> > - int type = 0; int other = 0;
> > - struct resource *p;
> > -
> > - res.start = start;
> > - res.end = start + size - 1;
> > + int rc;
> >
> > read_lock(&resource_lock);
> > - for (p = iomem_resource.child; p ; p = p->sibling) {
> > - bool is_type = (((p->flags & flags) == flags) &&
> > - ((desc == IORES_DESC_NONE) ||
> > - (desc == p->desc)));
> > -
> > - if (resource_overlaps(p, &res))
> > - is_type ? type++ : other++;
> > - }
> > + rc = __region_intersects(start, size, flags, desc);
> > read_unlock(&resource_lock);
> > -
> > - if (type == 0)
> > - return REGION_DISJOINT;
> > -
> > - if (other == 0)
> > - return REGION_INTERSECTS;
> > -
> > - return REGION_MIXED;
> > + return rc;
> > }
> > EXPORT_SYMBOL_GPL(region_intersects);
> >
> > @@ -1171,31 +1180,16 @@ struct address_space *iomem_get_mapping(void)
> > return smp_load_acquire(&iomem_inode)->i_mapping;
> > }
> >
> > -/**
> > - * __request_region - create a new busy resource region
> > - * @parent: parent resource descriptor
> > - * @start: resource start address
> > - * @n: resource region size
> > - * @name: reserving caller's ID string
> > - * @flags: IO resource flags
> > - */
> > -struct resource * __request_region(struct resource *parent,
> > - resource_size_t start, resource_size_t n,
> > - const char *name, int flags)
> > +static bool request_region_locked(struct resource *parent,
> > + struct resource *res, resource_size_t start,
> > + resource_size_t n, const char *name, int flags)
> > {
> > DECLARE_WAITQUEUE(wait, current);
> > - struct resource *res = alloc_resource(GFP_KERNEL);
> > - struct resource *orig_parent = parent;
> > -
> > - if (!res)
> > - return NULL;
> >
> > res->name = name;
> > res->start = start;
> > res->end = start + n - 1;
> >
> > - write_lock(&resource_lock);
> > -
> > for (;;) {
> > struct resource *conflict;
> >
> > @@ -1230,14 +1224,37 @@ struct resource * __request_region(struct resource
*parent,
> > write_lock(&resource_lock);
> > continue;
> > }
> > + return false;
> > + }
> > +
> > + return true;
> > +}
> > +
> > +/**
> > + * __request_region - create a new busy resource region
> > + * @parent: parent resource descriptor
> > + * @start: resource start address
> > + * @n: resource region size
> > + * @name: reserving caller's ID string
> > + * @flags: IO resource flags
> > + */
> > +struct resource *__request_region(struct resource *parent,
> > + resource_size_t start, resource_size_t n,
> > + const char *name, int flags)
> > +{
> > + struct resource *res = alloc_resource(GFP_KERNEL);
> > +
> > + if (!res)
> > + return NULL;
> > +
> > + write_lock(&resource_lock);
> > + if (!request_region_locked(parent, res, start, n, name, flags)) {
> > /* Uhhuh, that didn't work out.. */
> > free_resource(res);
> > res = NULL;
> > - break;
> > }
> > write_unlock(&resource_lock);
> > -
> > - if (res && orig_parent == &iomem_resource)
> > + if (res && parent == &iomem_resource)
> > revoke_iomem(res);
> >
> > return res;
> > @@ -1779,26 +1796,51 @@ static struct resource
*__request_free_mem_region(struct device *dev,
> > {
> > resource_size_t end, addr;
> > struct resource *res;
> > + struct region_devres *dr = NULL;
> > +
> > + res = alloc_resource(GFP_KERNEL);
> > + if (!res)
> > + return ERR_PTR(-ENOMEM);
> > +
> > + if (dev) {
> > + dr = devres_alloc(devm_region_release, sizeof(struct region_devres),
> > + GFP_KERNEL);
> > + if (!dr) {
> > + free_resource(res);
> > + return ERR_PTR(-ENOMEM);
> > + }
> > + }
> >
> > size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
> > end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1);
> > addr = end - size + 1UL;
> >
> > + write_lock(&resource_lock);
> > for (; addr > size && addr >= base->start; addr -= size) {
> > - if (region_intersects(addr, size, 0, IORES_DESC_NONE) !=
> > + if (__region_intersects(addr, size, 0, IORES_DESC_NONE) !=
> > REGION_DISJOINT)
> > continue;
> >
> > - if (dev)
> > - res = devm_request_mem_region(dev, addr, size, name);
> > - else
> > - res = request_mem_region(addr, size, name);
> > - if (!res)
> > - return ERR_PTR(-ENOMEM);
> > + if (!request_region_locked(&iomem_resource, res, addr,
> > + size, name, 0))
> > + break;
> > +
> > res->desc = IORES_DESC_DEVICE_PRIVATE_MEMORY;
> > + if (dev) {
> > + dr->parent = &iomem_resource;
> > + dr->start = addr;
> > + dr->n = size;
> > + devres_add(dev, dr);
> > + }
> > +
> > + write_unlock(&resource_lock);
> > + revoke_iomem(res);
>
> This is new, and not mentioned in the commit log, and therefore quite
> surprising. It seems like the right thing to do but it also seems like a
> different fix! I'm not saying that it should be a separate patch, but it
> does seem worth loudly mentioning in the commit log, yes?
This isn't a different fix though, it is just about maintaining the original
behaviour which called revoke_iomem() after dropping the lock. I inadvertently
switched this around in the initial patch such that revoke_iomem() got called
with the lock, leading to deadlock on x86 with CONFIG_IO_STRICT_DEVMEM=y.
This does change the order of revoke_iomem() and devres_add() slightly, but as
far as I can tell that shouldn't be an issue. Can call that out in the commit
log.
> > return res;
> > }
> >
> > + write_unlock(&resource_lock);
> > + free_resource(res);
> > +
> > return ERR_PTR(-ERANGE);
> > }
> >
> >
>
> thanks,
>
