* Jaeseon Sim <jason.sim@xxxxxxxxxxx> [230913 22:49]: > > * Jaeseon Sim <jason.sim@xxxxxxxxxxx> [230907 00:41]: > > > > On Thu, Sep 07, 2023 at 12:02:02PM +0800, Peng Zhang wrote: > > > > > > > > > > > > > > > 在 2023/9/7 11:49, Matthew Wilcox 写道: > > > > > > On Thu, Sep 07, 2023 at 12:39:14PM +0900, 심재선 wrote: > > > > > > > Use GFP_KERNEL on mas_node_count instead of GFP_NOWAIT | __GFP_NOWARN > > > > > > > in order to allow memory reclaim. > > > > > There are many paths that call maple tree's mas_node_count(). Some paths > > > > > cannot reclaim memory. > > > > > > > > Right ... but we should be handling the ENOMEM inside the maple tree and > > > > allocating some nodes with GFP_KERNEL instead of failing fork(). > > > > > > > > > > What testing did you do of this patch? In particular, did you try it > > > > > > with lockdep enabled? > > > I did power on/off test with this patch. > > > I did not try it with lockdep enabled. > > > > To accomplish the same result, but with a much smaller scope that will > > work with lockdep, I would suggest changing mas_expected_entries() to > > use mas_node_count_gfp() (which already exists) and pass in GFP_KERNEL. > > > > Since fork is the only current user of mas_expected_entries(), this > > won't break other users and we can deal with changing it for others if > > it is needed. > > > > If we do go this route, please add a note in the documentation about > > using GFP_KERNEL. > > > > Willy, does that work for you? > > > > Thanks, > > Liam > > Dear Liam, > Can I ask you the reason why mas_node_count is using GFP_NOWAIT | __GFP_NOWARN? Must users in the VMA space have complicated locking schemes which require no sleeping during a store operation. Most operations will drop the lock and re-try with GFP_KERNEL when using the internal lock (see mas_nomem()). > I wonder if other callers for mas_node_count might have similar issue. The external callers who need GFP_KERNEL are either using mas_store_gfp() or mas_prealloc to set up a store prior to taking a series of other locks. During a mas_prealloc() or mas_expected_entries() call, we set the MA_STATE_PREALLOC flag to indicate that there are nodes preallocated. This is to catch users who call mas_node_count() and require increased allocations when allocations should not be taken. You can see this flag directly below the line you modified. > > From your opinion, I'll post v2 patch as follows Thanks. Please test with lockdep but I don't see a nesting lock issue with fork and this change. > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c > index ee1ff0c59fd7..b0229271c24e 100644 > --- a/lib/maple_tree.c > +++ b/lib/maple_tree.c > @@ -5574,7 +5574,7 @@ int mas_expected_entries(struct ma_state *mas, unsigned long nr_entries) > /* Internal nodes */ > nr_nodes += DIV_ROUND_UP(nr_nodes, nonleaf_cap); > /* Add working room for split (2 nodes) + new parents */ > - mas_node_count(mas, nr_nodes + 3); > + mas_node_count_gfp(mas, nr_nodes + 3, GFP_KERNEL); > > /* Detect if allocations run out */ > mas->mas_flags |= MA_STATE_PREALLOC; > -- > 2.17.1 > > Thanks > Jaeseon
